3GPP 23.501 v20.2.0 — the document's own text
7.1.3 Network Function Service discovery
Taught in 5. Services instead of interfaces (The 5G system architecture, in depth), 3. How one function calls another (The 5G system architecture, overview).
A Network function (NF) within the 5G Core network may expose its capabilities as services via its service based interface, which can be re-used by CN NFs.
The NF service discovery enables a CN NF or SCP to discover NF instance(s) that provide the expected NF service(s). The NF service discovery is implemented via the NF discovery functionality.
For more detail NF discovery refer to clause 6.3.1.