Afaik modems do, it's the radio frontend that differs between continents. These differences are pretty slim nowadays and open market European phones will work i

Afaik modems do, it's the radio frontend that differs between continents. These differences are pretty slim nowadays and open market European phones will work in the US and vice versa. However the performance might be slightly degraded because some bands were prioritized over others that are supported for roaming. Also some combinations of bands for carrier aggregation might not work as well. And then you have OEMs (like Google!) who think it's smart not to enable 5G just everywhere but only for selected carriers, and you'll sometimes need a firmware update before you can utilize your phone fully. One big difference might be the mmWave support which is pushed by some US carriers and that'd be completely lacking otherwise (no antenna).