New iPhone se

If I buy a new iPhone se, will it work on both Verizon and ATT networks....say I try one like Verizon, don’t like it for whatever reason, can I then switch to att or any other carrier. I currently do not own a smart phone and don’t understand all this gsm and come stuff. I would like to get a new iPhone but don’t want to be stuck with a service I don’t like......Thanks for your help.

Jim