Sim Card Transfer

I am assuming you have an unlocked new phone with no supplied SIM. It’s how I buy them.

I am also assuming that your provider has not set you up with an eSIM if that is what you meant by embedded.

If my assumptions are correct you take the SIM from the old phone, pop it in the new one making note if the tray may be the other way round. Turn on the new one with the old one powered up with Bluetooth on and next to it. Start the setup from Hello and it will offer you the choice of direct Bluetooth transfer. (Quick Start)

If it has eSIM set up or comes with a SIM then you don’t need to swap the old SIM in.

If you don't currently have a 5G cell phone and iPhone 12 will be your first one, you need a new SIM card if you purchased a phone without a SIM card. Previous SIM cards are not provisioned for 5G service.

Before you start. You need it to activate the new phone

You’re welcome.

But they will work on 4G if 5G is not available in the area and not likely to be for another year at least. Personally I am waiting for my 4G contract to run out first.

Yes, it should work fine on 4G. But if the OP has 5G available to them, they'd be best advised to get a new sim card if the phone they purchased does not have a sim pre-installed.