Have you literally done zero research into this? The vast majority of merchants in the US, and nearly 100% in many other countries, accept Apple Pay. Doesn't that strike you as awfully high if they actually had to sign up for it with Apple? And add an entirely new payment processor to their operations?
Apple is not involved in any capacity with processing Apple Pay transactions when you tap your device in a business. A Visa card loaded on an iPhone is literally just a contactless Visa. Apple Pay = Google Pay = physical contactless card. One single industry-standard protocol.
For web/app transactions, a merchant has to set up Apple Pay explicitly (though it's still actually processed by the same parties as entering the card number) but for in person, they just need contactless payments enabled on their card terminal. No extra steps, parties, or fees.