The JS library is an interface to the HTTP API which handles functionality such as authentication, querying (and the likes) and more complex functionality such as real-time updates. I notice V2 also included auto-generated type defs for various table columns
Supabase now offers a few more features which integrate with the Postgres database - File Storage (s3), Authentication, Deno Functions, and Realtime (database change listeners). Each of these services is a standalone server and each has a corresponding JS library.
"supabase-js" wraps up the modular JS libraries into a single library for convenience