HOWTO? Permissions for user to access a single db

Hi,

Amateur question here :-{ Despite using Postgres for 15 years it's always
been locked safely inside a VPS with Hibernate on top powering a Java web
app. Each customer is on a separate VPS which typically has ~500k rows over
about 30 tables. Basic but very effective.

We now need to provide access to an associate company to a single database
(3 tables, ~10k rows) that our java app writes to (not JDBC, via
Hibernate). We have a nice SSH tunnel coming in, but they cannot view the
shared database (yes, I'm an amateur).

I'm just looking for beginners suggestions to get this db visible to this
user so we can continue our trials. They have their own Linux user login,
and their SSH access gives them access to port 5432 and nothing else. They
can see postgres, but no databases are visible.

Ubuntu 22.04
PG14
Their Linux user (say): "user2" / "theuser2linuxpwd"
Postgres user (say): "user2" / "myuser2pwd"
Postgres db they access (say): "mytransferdb"

In psql I did:
create user user2 with encrypted password 'myuser2pwd';
grant all privileges on database mytransferdb to user2;

I didn't think it was a pg_hba.conf issue because via SSH tunnel
they appear inside linux as if localhost (I think?).

After you stop laughing/crying, can anyone guide me?

Huge thx
-Damian