Primary meaning that where apps are opened when started.
Check for available connections names
xrandr
Screen 0: minimum 320 x 200, current 1920 x 2104, maximum 16384 x 16384 eDP-1 connected primary (normal left inverted right x axis y axis) . 2880x1620 59.96 + 50.00 . (...) DP-2-2 connected 1280x1024+0+1080 (normal left inverted right x axis y axis) 376mm x 301mm . 1280x1024 60.02*+ 75.02 . (...) DP-2-3 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 477mm x 268mm . 1920x1080 60.00*+
I want DP-2-3 to be the primary one and to the right of DP-2-2 so I run:
xrandr --output DP-2-3 --primary --right-of DP-2-2
The primary monitor is now on the right but the panel (lxpanel, start menu in microsoft terms) is suddenly on the other monitor now on the left. Thus right-click the panel I choose panel settings and there (geometry) as in the picture set "Monitor" to number "2". I also set "Alignment" to "right".
This can now be persisted via autorandr --save workstation --force