The "~2018 ... Form 1040 Individual Tax Return.tax2018" files are temporary files. Why is there still one left after I exit TurboTax?

Yes.  Started a couple years ago.  The ~ is a tilde file.   Those are temporary files that you normally don't see and normally are deleted.  But now Turbo Tax is leaving them in the folder in case something happens like your computer crashes.  Then you can go back.  Just ignore them.

This FAQ talks about the Tilde files,
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