It's anywhere close to open and requires vendors to seek approval from Apple for every implementation - Apple just has the market share to make everyone dance to their tune.
Apple Pay works on any terminal that supports NFC tap to pay payments. There are thousands, tens of thousands of terminals in the US that are not "Apple Pay" that you can use it on
Same here in australia. We had NFC payments ("tap and pay") since ~2006. Nearly a decade before apple pay launched in Nov 2015. For us, apple pay was like "oh, I can use my phone instead of my credit card? Neat."
People use it everywhere here, just because its easier than carrying around a card.
Most developed countries I've been to use NFC/Apple Pay. Where you see the use of QR codes is in China, Africa, and other countries that VISA/MC/AMEX hasn't penetrated (for either political or socioeconomic reasons).
Many countries in the world have local systems for quick, direct bank transfers encoded with QR codes to be used by local payment systems or local banking apps.