Chapter 13 of the Postgres official docs -> https://www.postgresql.org/docs/current/static/mvcc-intro.ht...
Concurrency Deep Dive talk @ PostgresOpen 2017 -> https://www.youtube.com/watch?v=ZxhBkBNxvR0
There is a wealth of good, informative, high signal/noise ratio talks at the PostgresOpen 2017, encourage you to check it out
And another famously good resource that's not quite related:
PostgreSQL internals through pictures -> https://momjian.us/main/writings/pgsql/internalpics.pdf