Restricted access on DataBases

Dear PG-masters!

We want to put more databases to one server, to "public" schema:
DB_A, DB_B, DB_C.
And users:
US_A, US_B, US_C, and Main_Admin.
We want to setup the environment.
Every simple user can access his database:
DB_A - US_A
DB_B - US_B
DB_C - US_C

They can't access other databases only theirs.

Main_Admin can access all databases.

I'm not sure how to do it perfectly.
We tried to remove "public" role, and add US_A to DB_A.
But the subobjects (table named "teszt") aren't accessable.

I can reown DB_A to US_A, but this revoke all rights from Main_Admin.

What is the simple way to we can avoid the access from another users, but
give needed rights to DB_[n] and Main_Admin? (Tables, Sequences, etc).

And how we keep this state later? For example: DB_A creates a new table.
Main_Admin must access this automatically...

I don't understand this area properly. For me the "public" means "access
for all users", which isn't good (DB_A vs. US_C).

As I think we can't mix the rights (Main_Admin = US_A + US_B + US_C...).

Thank you for the help. information, or an example!

DD