sql - How to make databases able to connect to each other?

Question

What are some ways that these two databases can [INSERT INTO] Between each other or use commands or display the data in the table of the two databases ?

Answer 1

Not directly, but there are ways...

What you're asking about is called "cross database queries".

Each database in PostgreSQL has its own system tables and ways of keeping itself organized. Queries between two databases can break this, even for databases hosted by the same database server.

But there are ways to achieve what you want

Single database, multiple schemas

Instead of two databases, you can run one database with two schemas. The keeps the tables, views, etc separated and easier to maintain, and allows queries between the two. It also allows security and data isolation for users who are only allowed to access one of the schemas.

You're actually already using a schema in PostgreSQL called "public"; adding more schemas simply extends this.

See the documentation.

Foreign Data Wrappers

Foreign Data Wrappers (fdw) allow you to "link" a schema (or just tables, if you prefer) in another database. See the documentation for CREATE SERVER, and this seems like a pretty clear blog post on the subject.

Note that Foreign Data Wrappers will allow you to link to other databases than just PostgreSQL e.g. Oracle, SQL Server, MySQL, and lots more. See here.