Welcome, krisdr, to Apple Support Communities!
On your Mac, your account name and password need not be related to those of your Apple ID, even though you, typically, associate your Apple ID with your Mac account, for various reasons and purposes.
However, it is easy to get confused by the various dialogue boxes asking for various authentications (user names and passwords).
When simply doing work on your Mac, you will, mostly, only be asked for an Administrator account and its password.
If you log into your Administrator account, typically, then the dialogue boxes will ask for Administrator authentication, and will have your Mac username as the default Administrator account to use.
You, then, simply fill in the password with the same password you use to log into your Mac.
Don’t even think about your Apple ID, in such cases.
None of this is a change, in any way, from earlier macOSs.
Big Sur works just like the earlier macOSs, in this regard.
By the way, do you explicitly log into your Mac, each time you power-up your Mac? Or do you have your computer set to automatically log you in, since you are the only user (so long as you don’t activate any other user accounts, including not activating the Guest account)?
With setting this Auto-Login feature, it can be easy for people to forget which of their many passwords is the correct password for their Mac User account.
If this is what has happened to you, you will be unable «to add additional users, update [your] security, NOTHING», not even «up date the Touch ID».
If this is what has happened to you, you can recover your Mac User account using your Apple ID (including your Apple ID password).