Regarding GSM, there are different standards for the radio frequencies for different parts of the world.
https://www.worldtimezone.com/gsm.html
About 7 years ago I bought an unlocked GSM phone specifically for European frequencies, it was 2G and 3G, it only had some of the USA frequencies so it did not work so well here in USA. So I just kept using my flip phone here in USA.
You can often tell the frequencies of the phone in the manufacturer literature from when you bought it.