CockroachDB is built from scratch in Go, uses Raft + RocksDB as a distributed key/value store, and then implements the Postgres data structures and wire protocol on top. Focuses on strong consistency and durability (which was the origin of its name). Can run active/active in multiple regions. Lacks full compatibility with Postgres.

Yugabyte is a multi-model database built in C++, uses Raft + RocksDB as a distributed key/value store, and then implements a proprietary document-store model on top called DocsDB. This is exposed as a Redis API, Cassandra API, and now PostgreSQL RDBMS interface by using the actual Postgres code for the SQL layer. Much higher compatibility with Postgres with all data types, most advanced SQL, and even some extensions that work without problems. Not as advanced on the distributed side and uses more of a primary/replica setup.

Both are great options for a distributed scalable RDBMS with Postgres.