Part of it is major, whether or not you apply specifically to some dept (eg, when there’s a separate College of Engineering or business school.) Because it’s an “institutional need” to balance the numbers in various depts. But remember that many depts that could have an easier admit (based on fewer applicants) are smaller, to begin with, have a smaller number of seats.
And absolutely, if you’re borerline for some competitive dept or program, they can wait to see what the RD pool offers.
I think many familes get confused. And when you find a video like Shapiro’s, on the surface, it seems to offer some clarity. But any time you’re talking highly competitive and holistic, you need to start with your own match. All points, all bullets. After that, it’s in their hands.