Hubbell-Raco 678 2-1/8-Inch Deep, 1/2-Inch End Knockouts, Wood/Metal Stud Bracket Welded 4-Inch by 2-Inch Handy Box

What size self drill screws should I use to mount these ?

How does this ts bracket work? are the frontmost tangs the "gauging tabs" and what is a "gauging tab"? how do i get it to be flush to a stud?

can bracket be removed?

Does this box have a grounding screw?