3GPP 23.501 v20.2.0 — the document's own text

8.3.5 User Plane for N19-based forwarding of a 5G VN group

Taught in 8. What actually travels on the wire (The 5G system architecture, in depth).

Figure 8.3.5-1: User Plane for N19-based forwarding

Details about the PDU Layer, PDU Session User Plane Protocol Stack are included in clause 8.3.1 and clause 8.3.2. The N19 is based on a shared User Plane tunnel connecting two PSA UPFs of a single 5G VN group.