No idea what they use. Things which come to mind: “ms paint” no lol… just kidding, though sometimes useable for arrows or so or circles
I do know about “smart draw” it has “diagram capabilities”.
And then there is also Microsoft’s visual draw or something… can’t remember it’s name but microsoft has something for it too. VisioCalc or something.
Both tools not super easy to use… so maybe there is something better out there…
Maybe adobe photoshop could be used for it… it has multiple layers which could be nice too… and decent text/font capabilities… easy re-positioning text/labels via layers.
Last but not least, perhaps even gui programming tools could be used to automatically draw something… but that’s probably a lot of work, manually with drawing tools seems faster.