how do i figure out which carrier the phone is from if i bought it from somebody locked and i can't get past the set up screen

Click on the link to read Apple's support article on how to unlock your iPhone for use with a different carrier. - How to unlock your iPhone for use with a different carrier - Apple Support - Only some carriers offer the ability to unlock iPhone for use on any carrier. Follow the instructions in this article to unlock your iPhone.

If you cannot get into Settings it sounds like you have more problems than carrier lock. Take the phone back to the seller and insist they do the things in this article or ask for a refund: What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Return the phone for a refund. It is going to be useless to you unless the seller performs these things for you:

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

Even if these things are done, you will have to find out who the carrier was in order to have the carrier lock removed on the phone. If the phone was stolen......likely.....the carrier will never remove the lock.

NEVER buy a used phone unless it powers on to the "hello" screen.

Like the others have suggested, the carrier the phone is locked is the least of your worries. You bought a used iPhone, which was not prepared correctly for a new user. If you can't get back in touch with whoever sold you this phone, it will be useless to you.

Read this --> The All Too Common SAD Reality of Buying … - Apple Community

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