If I think my iPhone has been hacked and I want to get a new phone with a new carrier how am I reassured that it won’t go to the new phone

If you did not jailbreak the phone, it has not been hacked. However, if you have concerns they have access to your Apple ID (which is separate from the device) please thoroughly follow Apple's recommended steps at If you think your Apple ID has been compromised - Apple Support

Would recommend using a fresh Apple ID for such new device. & additionally different security details.

I have come across this issue. There is no cure. Apple does not know anything about the preboot folder on you Mac iMac or MacBook Pro. No one has contacted me about a ransome attack problem. Good luck. Changing everything did not work for me.