Thanks, but that isn't what I was looking for. I gather you're in a later version of macOS, where System Settings – more like iOS – has replaced System Preferences? There is no Storage heading in Big Sur System Preferences > General; a search for "storage" goes to iCloud in Apple ID.
Which shows what apps are using iCloud. Which I know little about, as I've never used it (and don't want to). But have been noticing it grabbing stuff from my Mac and iPad, so I need to learn how it works so I can turn it off.
But what I'm looking for here is what System function was activated when the drive became overfull, so I can get the window pictured above again, and look around in it. I don't think it's any of the utilities in the Utilities folder.
I guess I could just stuff the drive with a lot of junk to get it to do the alert again. Maybe I'll do that.