What is the difference between "separating access control by DB" and "separating access control by schema"?

This is PostgreSQL12.
The PostgreSQL server uses AWS RDS.

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User: SQL Executor
Group—Indicates the range of access.
Assume there are DBs, schemas, and tables under the group.

Group A: User 1, User 2
Group B: User 3, User 4

In a table under a group A, a user 1 and a user 2 can execute an SQL command, and a user 3 and a user 4 SQL command cannot be executed.
The table under Group B allows users 3 and 4 to execute SQL commands, and users 1 and 2 cannot execute SQL commands.

At this time, the PostgreSQL concept is to set the group to

• Divide by multiple DBs
• Separate one DB into multiple schemas

I think there are two ways.

I think each user can control the DB or schema setting for each group by GRANT, but I would like you to tell me the advantages and disadvantages of dividing by DB and schema.

For a single DATABASE, you have to choose between creating multiple schemas (in this case, group A and group B) and controlling access, or creating multiple DATABASE (in this case, group A and group B) and controlling access.

"Basically, ""Separating a DB into multiple schemas"" is easier to operate, but that's all I can think of, so I'd like you to give me a clear answer on the basis other than that."