it, that...is not needed. 'Who is behind the disguise' should be accepted as an English answer, because that is an English speaker will normally ask the question. One might use 'that' if they are trying to emphasize a specific person, or not at all even when multiple are available.
תחפושת = costume
It is a common word.
especially on the Purim holiday, we wear תחפושות
מאחורי sounds like "nag ❤❤❤" in the audio. the lesson was "Tap what you hear" i I still can not hear the word מאחורי in it.