newbie questions re: Nokia 2780 flip phone and the sunbeam f1

I see everyone talking highly about the Nokia 2780 flip phone and the sunbeam f1 -- and it's very encouraging. I've been researching both and i have gotten my screen time on my iphone down to 1-2 hours (mainly for necessary texting for work).
I'm based in the US and currently use an iphone and don't fully understand how switching the sim card in and out works -- do I just buy one of these flip phones online, and I can put my sim card in them, and everything should be fine? do i have to ensure that they work with my carrier (t-mobile "essentials" with unlimited data) ? Is there anything that people recommend I know before buying one of these flip phones that I may be obviously missing?