Grant Fritchey explains a mechanism:
Let me start by giving you the short version of what MVCC is, and then the rest of the article explains more details. Basically, PostgreSQL is focused on ensuring, as much as possible, that reads don’t block writes and writes don’t block reads. This is done by always, only, inserting rows (tuples). No updates to an existing row. No actual deletes or updates. Instead, it uses a logical delete mechanism, which we’ll get into. This means that data in motion doesn’t interfere with data at rest, meaning a write doesn’t interfere with a read, therefore, less contention & blocking. There’s a lot to how all that works, so let’s get into it.
Click through for the dive. The pattern for MVCC is interesting, though quite different from pretty much any other implementation of concurrency management in a relational database system.