What you just mentioned is the SNAPSHOT Transaction isolation level strategy, where new updates are done on the Snapshot of the earlier one.

There is not a single way of handling concurrency, and no hard-and-fast rule, henceforth.

I have talked about the concepts in general, not taking in pictures any SQL Engine like Postres. Definitely, each one will have some differences, like Postgres does not support READ_UNCOMMITTED.

Please check your sources and confirm.