By being a ghost Danny’s form changes to match the dimension he’s currently in.

Sure it was a little difficult at first adjusting to experiencing the things in the 3rd dimension and occasionally the 1st or 4th, but he got the hang of it. There was just one little snag with the form shifting. His true form was a 2nd dimension body, so being summoned or forced into his true form in non 2nd dimensions made things very… awkward.

Young Justice didn’t know how to explain the “who framed Roger Rabbit” situation they were in, because they were pretty sure the cartoon cardboard standee in front of them was not some world destroying eldritch monster like the cult that they were supposed to stop promised to summon. At least they hoped they weren’t.

Completely unrelated, Robin definitely has contingencies for 2 dimensional creatures. Right?