Can anybody access 2020 return that was already submitted

Only you can (or someone to whom you gave your TT User ID and password).

I know that. Let me clarify....AFTER you enter your username and password why can't you access your 2020 return to make changes after you submitted and it was accepted.  The online version does not allow you to make changes.  If you buy the software and download it and NOT use the online version, you can go in at anytime and change your return without filing it.