Inside psql, you always have one active database, where you are “into”. By default it’s the one you connect to in the first place. When you run:

psql postgres

you’ll connect to the postgres database.

To switch database, use the \connect command, or \c:

Switch database in PostgreSQL

PostgreSQL will close the connection to the previous database you were connected to, and will connect to the new one you specified.

More database tutorials: