I can’t delete this and have no idea what it is… can someone shed some light?
Light orange = object is selected and “active” (usually the last selected object)
Dark orange = object is selected, but “inactive”
So, you most likely first selected the small object, then added the large one in the top and bottom area of your screenshot to the selection, thus making it the “active” object.
Then you tabbed into Edit mode. But: Only the “active” object changes into Edit mode, other selected objects “stay behind” in Object mode. That’s why you can’t select the dark orange object, as you are in Edit mode for another object.
Oh yeah I didn’t even notice how a second plane got created… well that’s strange, but much appreciated. Problem solved!