I think except the cryptography there is just trust calculations and encoding.

These two articles describe trust in detail:

https://www.linux.com/learn/pgp-web-trust-core-concepts-behi...

https://www.linuxfoundation.org/blog/pgp-web-of-trust-delega...

GPG esoteric options is also a good read:

https://www.gnupg.org/documentation/manuals/gnupg/GPG-Esoter...

Besides that... the RFC itself I suppose:

https://tools.ietf.org/html/rfc4880