T-Mobile iPhone 13 offer

Hello, I have a phone with each major carrier. My AT&T phone is an iPhone 10 and is lacking lots of goodies. I see that TMO is offering the iPhone 13 for "free". I know in most cases, both being GSM, that TMO and AT&T phones are generally similar. Is the TMO iPhone 13 mostly AT&T compatible?

I am also looking at the Walmart special coming out next week which still costs close to $400 so free is better.

The local TMO says they only stock the A2482 (universal version), and of course would be locked to the TMO network so I don't know if it would still work on AT&T with an AT&T SIM?

If it won't work out of the box on AT&T, I do know there are companies that offer an unlocking service, is that an option?

Thanks