in US, what stores or companies accept Apple pay?

It can be used most places. I'm not aware of any complete list as it would need to list most small local restaurants and stores in every city in the country.

The following from Learn where to use Apple Pay - Apple Support may help:

In stores

Use Apple Pay wherever you see one of these symbols.3 Or search for Apple Pay in Maps to see nearby stores that accept Apple Pay.

I have yet to find a Generalized list of locations accepting Apple Pay. There is an option in Apple Maps to search by vendors accepting Apple Pay

Also you can check if a specific business does accept Apple Pay by searching them on Apple Maps and scrollings to the amenities section.