[supabase cofounder] While we position ourselves as a Firebase alternative, it might be simpler for experienced techies to think of us as an easy way to use Postgres.
We give you a full PG database for every project, and auto-generated APIs using PostgREST [0]. We configure everything in your project so that it's easy to use Postgres Row Level Security.
As OP mentions, we also provide a few additional services that you typically need when building a product - connection pooling (pgbouncer), object storage, authentication + user management, dashboards, reports, etc. You don't need to use all of these - you can just use us as a "DBaaS" too.
[0] https://postgrest.org/