Can I use my iPhone Xr in the USA?

Yes. Some carriers have international plans and you could ask your domestic carrier about that. Alternatively you would need to use an eSIM or nanoSIM card from a compatible US carrier.

You could potentially encounter minor band incompatibility:

iPhone - Supported 5G and LTE networks - Apple

If you get the right SIM card, yes of course.

Im not sure I Google it and it said yes but something watch roaming