3GPP 23.501 v20.2.0 — the document's own text
5.17.7.2.2 Routing
Taught in 19. Living next to 4G (The 5G system architecture, in depth), 7. Roaming, other accesses, the edge and the outside (The 5G system architecture, overview).
The source RAN node sends a message to its core network node including the source and destination addresses.
MME uses the destination address to route the message to the correct AMF via N26 interface. AMF uses the destination address to route the message to the correct MME via N26 interface.
The AMF connected to the destination RAN node decides which RAN node to send the message to, based on the destination address.
The MME connected to the destination RAN node decides which RAN node to send the message to, based on the destination address.