I know that a 20-year old football player can play for his senior national team, but after playing in a match for the senior team can he return to play for the "youth" team?
Yes of course he can. As long as he is eligible in terms of his age, he can play for the under-21 team despite having played for his senior national team as well.
A recent example is Harry Kane. He made his senior international debut for England against Lithuania on 27th March, 2015. Later, in June 2015, he played for the England under-21 side at the under-21 European Championships.
Yes they can, and this happens rather often to young talents who are eligible for more than one national team. Until the player shows up at the senior team, he can still change his mind and play for another national side.
So in order to "snatch" a rising star, a national team manager might call him up on the senior squad typically for a friendly match.