Processes and files:

Every process has a PID:  Process ID number.

On Linux, every process has has a username/id and a groupname/id .  The username and groupname are 8 characters long.  The id's are numeric and are normally only used by the operating system itself.

The usernames/ids are stored in /etc/passwd

The point of the /etc/passwd is to associate human readable names with certain id numbers.

The groupnames/ids are stored in /etc/group .  The purpose of group id's is to give access to certain files to a group of users rather than a single user.

I have found, with what I do, that this is tedious method to for access, and so I use other ways.

On a linux system, every process has a userid and groupid.  (uid/gid).  A process can change its gid, provided that its user is a member of that group.  Even if a user is a member of twelve groups, it can only use one group at a time.

Files (and folders) have a set of permissions.  I think it's a total of 12 bits of permissions.  And we usually write them in octal.

400 : user read permission
 40 : group read permission
  4 : world read permission

200 : user write permission
 20 : group write permission
  2 : world write permission

100 : user execute permission
 10 : group execute permission
  1 : world execute permission

4000:
2000:
1000: are special permissions depending on the type of file

Read means you can access the information
Write means you can change the information in a file
Execute means you can execute the file as a program

The user/owner permissions apply to the owner of the file.  What can the owner do?  The group permissions apply if you are not the owner but your active group matches the group owner of the file.  The world permissions apply if you are not the owner nor does your active group match the file.  (Everybody else.)

A common file permission is 0644.  Owner has read and write.  Group has read.  World has read.