Opensource.com - https://opensource.com/article/18/4/how-add-users-group-linux
Linux offers versatile user/group structures. In this article, we will explore how to create and add users to a group.
In Linux, every process has an associated user, which tells you who initiated the process. Every file/directory is owned by a user and a group. Users who are associated with a file/directory can tell which user has access to that file and what they can do with it. A user who is associated with a process determines what that process can access.