Thanks, Etresoft.
The only reason I went searching for unusual aspects of the system is because of the many other problems I encountered with 15.0 release. 15.0.1 seems to have fixed all of them except for one remnant left because of Safari not working. I made the mistake of downloading Chrome (ended up using DuckDuckGo (for the interim), which is pretty fine indeed). Chrome, though, left some remnants when I deleted it that I find in "Login items and Extensions". Google LLC and Google Updater remain in the "Allow in the background". I have them both turned off. I am beginning to think that Google LLC and Google Updater may not even exist any longer. I can't find a trace of them. But, I really wish I had never downloaded chrome. I'd love to assure myself they are really gone.
There are a couple of other quirks that are new and won't go away. Nothing that I'm too concerned about, but still...
You're right. I should have written 256 GB.
The thing is that System Data seems to adjust to the size of storage available. I wish they would just say that.
If you read the info on System Data, the way Apple explains it is that System Data is not available to the end user. I appreciate the insights and, if I really hit a wall, I will check into it.
Are you sure that it is just 'other data'? I am beginning to think that may be where virtual memory resides. That would make a lot of sense.