Where could I find an explanation of all settings in "Edit > Preferences" - especially the Lightroom

If you can’t find an explanation in Lightroom Classic Help, there is a brief explanation in the Lightroom Classic Missing FAQ book by Victoria Bampton (a paid book).

But it’s actually a simple explanation. Because Lightroom Classic stores all originals locally, it downloads anything it sees in cloud sync (Lightroom Photos), like photos imported into the Lightroom app on phones and tablets. So where on your computer is it going to download all those files? That’s what the option controls.

This is what I do: I set the synced file Location to the exact same folder that I set in the Import dialog box (Destination panel) for photos cataloged from camera cards. That way, no matter where I take a photo or how it comes into Lightroom Classic, it gets organized into the same single set of date-organized folders, which for me are on an external volume.