How do I properly find out what my AGI for 2020 is when I have both a W2 and W2-C for that year?

If you are looking for your AGI in order to e-file, don't waste your time.   E-filing is closed.  You have to file by mail, and you do not need AGI to file by mail.

But....your AGI is never on your W-2 anyway.   It is on line 11 of your tax return.

When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s.  Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.

Federal and state returns must be in separate envelopes and they are mailed to different addresses.  Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.