Postgres is a relational database server with extension hooks for storage formats and access methods. The reason it's the default SQL engine for new projects is the combination of MVCC, PL/pgSQL, and a permissive license.
Subtransactions are cheap up to a point, then VERY expensive — the SLRU buffers become the bottleneck. If you find yourself with nested savepoints in a hot path, audit them.
Streaming replication via WAL shipping is the default. Logical replication via decoded WAL records is more flexible but the publication / subscription dance has more moving parts.