Change arrangement of iPad used as Sidecar Display
Sidecar has no settings to change monitor arrangement. You only enable the iPad as an additional display in the Mac's System Preferences.  You still have to go to "Displays" in your Mac's System Prefs to change the arrangement of the iPad as a display relative to your Mac.

After (2) days of dragging things in the opposite direction to the iPad display it finally occurred to me to check Displays...  D'Oh!