Most of the changes aren’t relevant to upstream - a lot these companies seem to be using Postgres as a frontend over some kind of secret sauce, usually related to custom storage or deployments or the like. Postgres seems very modular and pluggable that way.
I assume for the common Postgres core changes everyone is already incentivised to contribute changes back upstream - the companies are banking on Postgres popularity can need it to have an excellent reputation even outside of their specific customised offering.
Another recent one, AlloyDB from Google Cloud. Looks pretty interesting/promising. Sort of like their competitor to AWS Aurora, built on top of Postgres, with the added ability to handle OLAP workloads well (along with apparently great OLTP performance).
TimescaleDB and Citus are notable too.
But yeah, if you’re building a DB product, starting with Postgres and customizing from there seems like an excellent approach.
