Please explain the permissions for the $/$tmp directory. What is the significance of this directory?

As user A$.($created above$),$ what permissions need to be changed so that an empty file $($using touch command$)$ for User B$?$ Please provide the permissions that need to be altered including what file$($s$)$ or directories are included.

As user B$.($created above$),$ what permissions need to be changed so that an empty file $($using touch command$)$ for User C$.$ Please provide the exact permissions that need to be altered including what file$($s$)$ or directories are included.

As user C$.($created above$),$ what permissions need to be changed so that an empty file $($using touch command$)$ for User A$.$ Please provide the exact permissions that need to be altered including what file$($s$)$ or directories are included.

What are the default permissions when creating a file?

Please provide the exact list of files that are used within the login process.