In Unix and Linux there are two levels of permissions: standard user and superuser (usually called root).
The standard user has access only to the files that he has permissions for, by ownership, group membership or ACL.
The superuser has permissions to everything (we'll ignore stuff like SELinux and the like for this answer) without limits within the userspace.
When a process has an effective or real uid being root and tries to access a file, is it still subject to permission bits of the file? Or like the quote says (if I understand it correctly), having euid root will override the permission bits of the file?
when the owner of the file is not root and its group doesn't match the effective group or supplementary groups of the process, will the permission bits of the file for "others" apply to the process? If the file doesn't allow others to read or write or execute, can the process still read or write or execute?
when the owner of the file is root, and the permission bits of the file for "user" doesn't allow read, write or execute, can the process still read, write or execute the file?
Is it possible to make a process with effective or real uid being root unable to access a file? For example, by changing the permission bits of the file?