Mode selection is a feature that allows the system to choose the best mode for an activity based on the mode priority and the available capacity of the resources. A mode is a combination of a resource and a formula that determines the duration and quantity of an activity. An operation can have multiple activities, each with multiple modes. Mode selection can be used when an operation contains several activities, each of which is assigned several modes for different resources, and the system needs to determine which resource to use for each activity. Mode selection can also be used when an activity has several modes for the same resource, and the system needs to determine which formula to use for the activity. Mode selection can be performed during the planning explosion or during the interactive planning. Reference: Mode Selection | SAP Help Portal, Mode Selection | SAP Help Portal

Collective orders are used to represent multilevel production structures for materials produced in-house, where the manufacturing order for the highest material in the structure automatically generates manufacturing orders for the semifinished products in the structure. This allows for an integrated view of the production process and avoids unnecessary placements in storage or removals from storage between production levels. Collective orders can consist of planned orders, production orders, or process orders, and each order has its own order number. Collective orders can be processed with or without automatic goods movements, depending on the business scenario. Reference: Collective Orders | SAP Help Portal, Collective Order | SAP Help Portal, Combined production order Vs Collective production order in SAP PP

Before you create a BOM, you need to check if the material type of the header material is allowed in the BOM usage. The material type defines the attributes and functions of a material, such as whether it is a finished product, a raw material, or a trading good. The BOM usage defines the purpose and application of a BOM, such as production, engineering, or sales. Only certain combinations of material types and BOM usages are valid in SAP S/4HANA. For example, you cannot create a production BOM for a material type that is not relevant for production planning. Reference: [SAP S/4HANA Production Planning and Manufacturing Certification Guide], page 63; [SAP Help Portal: Material Type].

The duration of an operation for production is determined by the combination of data maintained in the routing and the work center. The routing defines the sequence of operations, the work centers, and the standard values for each operation, such as setup time, processing time, and teardown time. The work center defines the capacity, the formulas, and the scheduling parameters for each operation, such as base quantity, operation rate, and queue time. The system calculates the duration of an operation by multiplying the standard values from the routing by the factors from the work center formulas and adding the scheduling parameters from the work center. For example, if the routing specifies a processing time of 10 minutes per unit and the work center formula is processing time = (processing time / base quantity) * operation quantity, and the base quantity is 100 units, then the duration of the operation for 50 units is (10 / 100) * 50 = 5 minutes.
The bill of material (BOM) (C) and the material master (D) are not data objects that determine the duration of an operation for production. The BOM defines the components and the quantities that are required for producing a material. The material master defines the general and specific attributes of a material, such as the material type, the unit of measure, the MRP data, and the production data. These data objects are relevant for planning the material requirements and the production costs, but not the operation duration. Reference:
https://help.sap.com/docs/SAP_S4HANA_ON-PREMISE/c00ad2cac7be4ef29a4461ba114e4f91/051ebf53d25ab64ce10000000a174cb4.html
https://learning.sap.com/learning-journeys/manage-production-orders-in-sap-s-4hana-manufacturing/scheduling-production-orders_cc390d5f-b87c-4c4d-9472-8bb10ea4f476

You can select the following standard item categories inside a bill of material (BOM):
Variable-size item: This item category is used for materials that have variable dimensions, such as pipes, wires, or fabrics. The system calculates the quantity of the variable-size item based on the formula and the dimensions entered in the BOM. You can also use the variable-size item to define a scrap percentage for the material.
Non-stock item: This item category is used for materials that are not managed in inventory, such as consumables, services, or subcontracting items. The system does not create any reservations or stock movements for the non-stock item. You can use the non-stock item to trigger purchasing requisitions or purchase orders for the material. Reference: Bill of Material Item Category; Bills of Material in Production Planning

The material type is a key attribute that classifies materials according to their characteristics and determines how they are processed in the system. The material type influences the following aspects of material management:
Which material master screens appear and in which sequence: The material type controls which fields are required, optional, or hidden in the material master record. It also determines the order in which the screens are displayed when creating or changing a material master record. Different material types may have different views and fields depending on the business requirements. For example, a raw material may have a purchasing view, while a finished product may have a sales view.
Whether the material is produced in-house, procured externally, or both: The material type defines the procurement type of the material, which indicates how the material is procured. The procurement type can be in-house production, external procurement, or both. The procurement type affects the planning and execution of the material requirements. For example, a material with in-house production procurement type will generate planned orders, while a material with external procurement type will generate purchase requisitions or schedule lines.
The material type does not influence the following aspects of material management:
Which document types and class categories are allowed: The document type is a key attribute that classifies documents according to their business purpose and determines how they are processed in the system. The document type controls the number range, field selection, and status profile of the document. The document type is independent of the material type and can be used for any material type. For example, a purchase order document type can be used for any material type that is procured externally. The class category is a key attribute that classifies classes according to their application area and determines how they are processed in the system. The class category controls the characteristics, object types, and inheritance rules of the class. The class category is independent of the material type and can be used for any material type. For example, a material class category can be used for any material type that has characteristics.
Which plant-specific and plant-independent statuses are allowed: The status is an attribute that indicates the current condition of an object in the system. The status can be plant-specific or plant-independent, depending on whether it applies to a specific plant or to all plants. The status is independent of the material type and can be used for any material type. For example, a material can have a plant-specific status of blocked for quality reasons, or a plant-independent status of discontinued. Reference: Material Type | SAP Help Portal, Material Master (LO-MD-MM) | SAP Help Portal, Document Types | SAP Help Portal, [Class Category | SAP Help Portal], [Status Management | SAP Help Portal]

A multiple bill of material (BOM) is a type of BOM that allows for different combinations of components or raw materials for the same assembly or subassembly. The reason for different combinations of components in a multiple BOM could be the manufacturing date or the lot size. The manufacturing date determines the validity period of the BOM, which means that different BOMs can be valid for different time intervals. For example, a BOM can be changed due to a design modification or a change in the availability of components. The lot size determines the quantity-dependent explosion of the BOM, which means that different BOMs can be used for different production quantities. For example, a BOM can be optimized for different batch sizes or different production methods. Reference: Multiple BOM, BOM Explosion

Backward scheduling is a method of determining the order dates for components from dependent requirements by starting from the requirement date of the finished product and working backwards through the production structure. Backward scheduling considers the following time elements:
Operation duration: This is the time required to perform an operation on a resource, such as a work center or a machine. Operation duration is influenced by various factors, such as the quantity, the lot size, the setup time, the processing time, the teardown time, and the operation scrap. Operation duration is subtracted from the requirement date of the finished product to determine the start date of the operation.
In-house production time: This is the time required to produce a material in-house, from the start of the first operation to the end of the last operation. In-house production time is influenced by various factors, such as the routing, the work center, the production version, and the scheduling margin key. In-house production time is subtracted from the start date of the first operation to determine the order start date of the material.
Backward scheduling does not consider the following time elements:
Total replenishment lead time: This is the time required to procure a material externally, from the creation of the purchase requisition to the receipt of the goods. Total replenishment lead time is influenced by various factors, such as the planned delivery time, the goods receipt processing time, the release time, and the scheduling margin key. Total replenishment lead time is used in forward scheduling, not backward scheduling, to determine the order dates for externally procured materials.
Planned delivery time: This is the time required to deliver a material from the vendor to the receiving plant. Planned delivery time is maintained in the purchasing info record or the material master record. Planned delivery time is a component of the total replenishment lead time, which is used in forward scheduling, not backward scheduling, to determine the order dates for externally procured materials. Reference: Backward Scheduling | SAP Help Portal, Scheduling | SAP Help Portal, SAP S/4HANA Production Planning and Manufacturing Certification Guide, page 77-78.