New iPhone activation on current carrier & SIM cards installed in unlocked phones?

I went to AT&T last week to upgrade my iPhones. I also told the CSR at the store that I wanted to keep my current wireless plan (12GB mobile share w/ rollover). I know it’s old but I get a 19% discount through my job & only pay $107/mo total for 2 lines. AT&T suggested I go to the Apple store & buy the phones, then put the SIM cards from my 2 iPhone 6’s into the iPhone 11’s.

I went to the apple store today and purchased 2 iPhone 11’s in full. They are unlocked and the receipt says that no carrier was chosen. During the purchase the Apple CSR said that there were SIM cards in the phones already and all I had to do was call AT&T, give them the IMEI & ICCID #’s on the back of the box and they would be able to activate the phones & the new SIM cards that are in the new phones. He said I could switch the old SIM cards to the new phones but that also the old SIM cards may become problematic because of their age and that I should just activate the new ones that came with the phones. So my questions are:

Do unlocked iPhones come with some kind of “unlocked/carrier free” SIM card that can be activated with any carrier?

Or are these unlocked phones with AT&T SIM cards in them that need to be activated?

If they are unlocked SIM cards for any carrier, should I activate them or just switch the old ones? The key is to keep my current plan & not be suckered into an upgrade by activating 2 new SIM cards.

I didn’t get a chance to open and check out the new phones today so I’m still using my old ones until I get some free time but any info before I get to it will be helpful. Thanks for your time.