sim card

You can use the sim card from the iPhone 11 Pro Max without issue assuming it still has active service on it. If you transferred the service to the new SIM card during purchase of the iPhone 12 Pro Max, then you will want to use that new SIM card rather than the old inactive one as it is the one that holds your number and service now.

That…. Depends.

The physical SIM will swap.

The answer to that depends on your carrier, and how the iPhone was ordered. If the new iPhone was ordered from Apple and with the identification of your current carrier, you’ll probably be able to perform the transfer electronically. Some will pre-populate the new iPhone with an eSIM. Some carriers will require a physical SIM swap. Some carriers require a different SIM for 5G access. Others will perform the swap for a fee, and after a discussion with carrier support.

Carriers seemingly can’t do things… simply.

The contents-migration during iPhone setup will usually tell you what your options are, and what should happen for your carrier and your situation and your cellular plan. Put differently, perform the migration during your new iPhone setup, and follow the instructions provided.