For iPhone 12s and 11s, they come in three different models each, designed for different countries. It's important to get the right one for you.
There are too many bands in the world so they will all roam but coverage will be different.
Basically, the US model has a CDMA modem for rural AT&T and Verizon. And they support special LTE bands 71 for t-mobile and 14 for AT&T. They don't support t-mobiles special 600mhz. This is actually very good. It used to be that the Verizon phone was different from the ATT phone so it's nice there is only one US model.
The China model is like the US but has two physical sims. The US one has an Esim but also an extra sim slot so you can use it with two phone numbers. Good for international.
Finally, the rest of the world model lacks the US extras but has more LTE bands for Europe. And a good band for LICs (Lower-income countries).
— Read on www.pcmag.com/news/beware-there-are-3-versions-of-each-iphone-11