I'd also add https://postgres.fm to the list if you are looking for a Postgres in production podcast that explains a lot of things. I always feel like I learn something new even if it's just something small.
Since Postgres 12, there's been support for “Pluggable Storage” where you could even define different storage engines for tables individually. Think of the Oriole extension which bought by Supabase in April 2024.
I don’t think “DB OS” is a standard term, but what I believe the commenter means is that postgres has evolved to become an incredibly mature, well built set of DB building blocks that are exposed via extension APIs etc to implement whatever kind of database you want.
Sure, there’s the core row-major RDBMS you get out of the box, but could easily turn that into a distributed column-oriented analytics data warehouse if you wanted to.
Let me do a lazy copy-paste "The phrase "Postgres is the database OS" is often attributed to Bruce Momjian, a co-founder of the PostgreSQL Global Development Group and a prominent figure in the PostgreSQL community. He has frequently used this phrase to highlight the extensibility and versatility of PostgreSQL, comparing it to an operating system in terms of its ability to support a wide range of functionalities through extensions and customizations."
I'm not at all affiliated, just a happy consumer of their material.