Does my iphone 12 have a special sim card?

Because of 5G service, you should use the new sim card which came with your phone. If you didn't identify your carrier and phone number when you ordered the phone, you'll need to activate the new phone through your carrier with your phone number. If you bought the phone from Apple and you identified your carrier and phone number, just turning the phone on will activate the the phone to your number.

The Apple Store and AT&T told me to use the sim in the new phone. Whether there is validity to this or not, it worked out perfectly for me, so since Apple and my carrier recommended it, that's what I've recommended to others.

And T-Mobile told me to just use my old SIM card, that there was no such thing as a 5G SIM card.

My phone is also working perfectly.

I have no doubt.

There is no special SIM card for the phone.

Use your old SIM card if you have T-Mobile.

AT&T and Verizon claim you need a 5G sim, though no such thing exists.