Saxman Said:
"I still have no idea what you mean by an 'app password', [...]Just like I don't understand why when I try to log in to this site, two different passwords pop up, both with my Mac ID email, and when I try to see which is in my keychain, it's impossible to figure out which it is, as it shows a dozen or more, all with my Mac email address!![...]"
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About the App Password:
The app password allows you to access your eMail provider's server for user on a third-part app (i.e Apple's Mail app). The incoming and outgoing servers much have their own protocols. So, who is you eMail provider?
Reinstalling the macOS:
Another option would be to reinstall the macOS. Perhaps an install was not 100% successful? Sometimes, when major changes are made to your Mac(i.e. update installations), system settings (i.e. for the fan) become manipulated, technically. So, when reinstalling, nothing would be deleted. It's just that items that have become corrupted or deleted, would be replaced or reinstalled back to where they should be. Go Here: How to Reinstall macOS from macOS Recovery - Apple Support
Important: Be certain to back up your Mac prior to making any major changes to it, successfully as au update. In doing so, you with have something to restore store your Mac from from, should anything go wrong with the reinstall.