3GPP 38.300 v19.3.0 — the document's own text

4.4.1 User Plane

Taught in 2. The six working groups, and where the line between them runs (TSG RAN — the group that specifies the radio, in depth), 5. The radio protocol stack, sublayer by sublayer (TSG RAN — the group that specifies the radio, in depth), 5. The layers a packet falls through, and the three states a device sits in (TSG RAN — the group that specifies the radio, overview), 3. What NR is, read off the document itself (TSG RAN — the group that specifies the radio, quick start).

The figure below shows the protocol stack for the user plane, where SDAP, PDCP, RLC and MAC sublayers (terminated in gNB on the network side) perform the functions listed in clause 6.