On a related note, a difference in Postgres is that both Repeatable Read and the higher Serializable Isolation levels are both using Snapshot Isolation under the hood.

I've been trying to grok this stuff for a few years and it's good to keep in mind that the implementations of various isolation levels between vendors can vary significantly.

I've found the resources on Jepsen very helpful as well if you want to go down a rabbit hole :)

https://www.postgresql.org/docs/current/transaction-iso.html

https://jepsen.io/consistency

https://jepsen.io/consistency/models/snapshot-isolation