iPad 10.2 (7th Gen.) without SIM card

I recently replaced my old iPad with a new refurbished one... I was going to move the SIM card from the old one to the new one when I noticed that the old one did not have a SIM card. I mostly use this iPad with WiFi but I do also use it when I'm out and about and don't necessarily have access to WiFi, so I was worried that it wouldn't work until I got a SIM card from my carrier (AT&T) -- and very puzzled as to how my previous iPad was able to work while there was no available Wifi. To verify what would happen if I had no Wifi, I turned off Wifi and turned Cellular ON. My access to websites continued to work normally.

Can anybody explain how it is possible that I can use cellular data if I don't have a SIM card ? Without a SIM card how does AT&T get to know that I used cellular data so they can charge me ?