(your “how much sand” question)
Every bunker shot is different . It depends on the lie and the conditions and kind of the sand. There too many situations a to mention. You could be in very soft sand and buried… all the way to hard-pan or no sand to speak of, sitting right on top. How much sand you take, depends also on how you want the ball to come out and react. How much sand also depends on how much bounce you have on your SW, and how much you’ve opened/closed the face. The more you open it, the more it slides through the sand vs. closed which creates a “dig sole” and pulls the face down into the sand to help gouge it out.
(Your “hit ball first” question)
In general… you hit ball first (if you can), when you’re farther away for a clean hit… but once you’re too close to the green, you hit sand first, use the sand to “lift” the ball out. If it is hard-pan you still want to hit ball first with a square face, or the club will bounce off the hard-pan into the ball an skull it.