The entirety of a packet at one layer becoming the payload section at another layer is known as

The best answer to this question would be encapsulation

Payload data is part of a packet that is within a TCP or a UDP. It is the actual or essential data that is being carried within a packet. For instance, a layer 2 Frame has a payload of a layer 3 packet and a layer packet has a payload of layer 4 Segment. This simply can be called encapsulation. Each layer in the examples mentioned builds a PDU by adding a header containing control information to the PDU.