In my environment variables I have this:
SIGIL_DEFAULT_TEMP_HOME=C:\_AppData\Sigil\Temp
But this is what I see when I open a file in an external editor
I think I used to see C:\_AppData\Sigil\Temp\Sigil.xxxxxx\OEBPS\Text\bla h-blah.blah
I don't where I got the envar name SIGIL_DEFAULT_TEMP_HOME. It's not a name I would have invented, a) because there'd be no point, b) I prefer short envar names because of MS's tiddly dialogue box - even though I don't normally use it
I can't find any doco for Sigil's Environment variable usage - looked in Wiki and Sigil_User_Guide_2019-06-25.
BR