What is the scope of the database user?

Question

linux

On Linux
Let's say I have Postgres, MySql, Oracle, MongoDb + 5 more different DBMS on one machine.
1) If I create a user user1 in each of these DBMS, then it will be different users for each DBMS? Will they not see each other? And they won't overwrite each other somehow?
2) And if I already have user1 in the system, and I also create user1 in these DBMS, then the system user and the DBMS user will not be in conflict? Will they overwrite each other?
Is the user created in the DBMS visible in the system?

Answer 1

Each system (both DBMS and OS) has its own access control mechanisms. They are not related to each other in any way.
For example, there are no users in PostgreSQL at all. There it is called a login role. And user is a property of the role.
The operating system has its own problems. The OS user is some entity on behalf of which programs are launched, for example, a DBMS. And this program may not have rights to create new OS users at all.