New iPhones bought from Apple that are unlocked “connect to any carrier later” work on all the networks in the us. Once upon a time, there was an “unlocked” phone - meaning you could change the sim and the phone wasn’t locked to a contract. But you still had to match the phone to the major carrier. For example, an att phone could be unlocked, and then used on straighttalk (becasue straighttalk resold att network). But it wouldn’t work on Verizon or T-Mobile because they were different networks.

That’s not a thing anymore with iPhones and hasn’t been for a long time. An unlocked iPhone can be used with any carrier that supports esims.

If your old phone is still on a contract - you may not be able to transfer the phone number, or have to request an unlock, or any other shenanigans. But the new iPhone will still work on whatever network you take it to.

Ideally, your contract is done, you buy new unlocked iPhone, you take it to your existing or a new carrier, you say “I bought a new unlocked phone, I want to set it up new, and I want you to transfer my number” a prime time carrier will just make this happen for you. A reseller can be a little more of a pain in the arse.

Personally I’ve been happy with the prepaid plans from straight talk - despite their setup process sucking. If you call them and get a person to help it goes pretty smooth. And the service is indistinguishable for a much cheaper price once it’s setup. I’m pretty sure this goes for most resellers.

Good luck - you’ll be fine!