Yugabyte uses actual Postgres code for the query layer, on top of data persistence (distribution/replication) handled by DocDB document store, which itself is a layer on top of RocksDB: https://blog.yugabyte.com/how-we-built-a-high-performance-do...

CockroachDB (aka CRDB) is completely custom and compatible with Postgres wire/datatype protocols, which operates directly on its own key/value store called Pebble (but originally was also RocksDB): https://www.cockroachlabs.com/blog/distributed-sql-key-value...

Both systems are foundationally the same SQL-on-KV but implement it very differently.

As opposed to? Postgres is “just” a Postgres front end on top of a kv store.

I don't think that's really true. Which part of postgres would you describe as a KV store, compared to what cockroach does with RocksDB?