Apple collects 0.15% of all credit card transactions made through Apple Pay for US-based cards. The percentage is higher for the Apple Card, of course.
Of course they’ll take 0. You do realize that Google is in the business of harboring as much data about you as possible. It’s much more worthwhile for them to know what you’re buying than for them to take a cut from using Google pay.
They take a cut from the banks (which may or may not be passed onto the retailer, I'm not sure).
Individual retailers do not form an agreement with Apple. Hell, retailers often don't even explicitly and individually support Apple Pay, it just comes through accepting standard contactless payments.