Hi,

Tried to post a pic, but won't let me since I am new...

Total newbie to bikes here. My son join a MTB team and I swapped his tires for him. When I loosened the rear axle, the derailleur came off. Appears it was held in place only by the axle nut and I do not see how I can reattach it with confidence. Youtube/internet searches came up empty other than to make me think it had not been attached correctly to begin with? When I took the wheel off again and lined up the hook where it could thread to the frame, the axle slot does not line up with the hanger (?).

He has practice tonight and I would appreciate any help!

At the basic/economy end of the derailleur game, yes they do fit under the axle nut.
the frame has no integrated hanger /dropout on the right/drive side.
This piece does that and lets you select different rear derailleurs ..

bolt , and half-moon shaped nut, holds it in place when wheel is removed..
Making a QR wheel axle practical.