Good questions. As to (1), it’s geared primarily to the POTUS and those who work closest to him. One of Congress’ (modern) duties is a general oversight function, so a Pentagon janitor won’t be invoking executive privilege if called to testify.
As to (2), it doesn’t really matter. Ultimately, the privilege attaches to the office of the POTUS.
As a general rule, any claim of privilege requires at least a minimal description of the documents which the privilege is supposed to attach to. Otherwise, it’s impossible (1) for anyone else- including the court- to determine whether the privilege applies or not; and (2), for a court to determine whether the need for disclosure outweighs the privilege, in cases where it isn’t absolute.
In terms of executive privilege, the documents must be identified specifically enough to determine whether they relate to a specified executive function under Article II of the Constitution. The separation of powers doctrine means that courts defer to the executive and legislature in matters which the constitution says they’re in charge of (foreign policy and warmaking are common examples).
You may find the discussion in Section IV of the US v. [Richard] Nixon opinion enlightening. It’s about halfway down.