There are 32 areas currently in the game, and a large majority of them are available in act 1 of the game. All areas are accesible in Act 3, though a large majority of them are changed from Act 1 and 2.
Accessible after defeating the Last Judge or Phantom, it is mostly composed of the Citadel, along with a few other areas.
Only accesssible after a long questline and completing every wish in the game, it is a very hidden portion that has multiple unique enemies, bosses, and the true ending of Silksong.