How 'real' are the reality shows?

Hey guys there are so many reality shows. But nowadays, I really kind
of doubt these shows. You see, in American Idol outside of the
audition place, there are tens of thousands of contestants waiting for
audition. But how on earth, the judges audition every single one of
these tens of thousands of people?
And why after these competitions, the winners are not as popular or
famous like real singers or actors?
Can the dialogues in the reality shows be pre-rehearsed?
Nowadays, I really kinda doubt these shows.