Network carrier

Go to Settings > General > About.. scroll down, might be shown under 'Carrier Lock'? My phone is not carrier locked, so it says 'No SIM Restrictions', but it does show AT&T under the next section 'ESIM'.

If using a physical SIM and it has the original one.. You could pull it out and look at it, should say the carrier name.

The carrier is the company that you pay every month for your phone service.