System font mysteriously replaced on 2019 iMac? Not sure how this happened or how to fix it.

Hi,

I have a strange issue on my 2019 iMac. I updated to Big Sur last month, but a few days ago a font took over my computer. I don't know if it's a system font issue or what. I'm trying to resolve it myself before calling AppleCare, but I'm not sure what to do to fix it or how it happened in the first place.

Somehow, the factory font displayed everywhere on my Mac (login page, finder, in apps, Safari, Chrome, web pages...even this dialogue box I'm typing in) was mysteriously switched to a midcentury modern typeface. Please see screenshots.

I have tried looking for a setting in System Preferences, validating all fonts in FontBook, deleting duplicate fonts, restarting my computer, unplugging it... nothing is working.

Does anyone know how this could have happened and what I can do to fix it? I can't even view websites online without seeing this typeface on sites. Please see screenshots for examples.

Thanks for your help. I am at my wits end.

Mac Login page

Finder sidebar:

Font replaces default font on Web Pages

File names on Mac: