I’ll absolutely admit knowing very little about the nuances of payment networks in the US, particularly with contactless —- but my experience (in Australia) is that payment terminals that support contactless inherently support Apple/Google/etc Pay, even if the payment terminal is unaware of those things, in which case the transaction will work the same as a contactless card transaction (e.g. transactions >$100 require PIN).

You don't have to have an agreement with Apple or Google to accept Apple Pay or Google Pay. If the customer holds a Visa then the phone presents a Visa to the reader. There is no special thing you have to do to accept those payments.