Do you wear underwear over jockstrap?
Not usually. Usually when you have a jock on, you don't wear
anything else with it, just your athletic gear on top of it.
However, if you are wearing compression shorts, sliding shorts,
hockey shorts, etc., you should wear those over the jock. Some
people (inexplicably) wear a jock under a swimsuit, too. If you're
wearing the jock for running or track, you just wear your normal
shorts over the jock, no underwear.
You can wear a lot of modern underwear (briefs, gripper jams,
etc.) INSTEAD of a jock, particularly if the underwear fits closely
and is supportive.