After the Boss of an area a door can spawn Wich leads to the alternative chapters.

DownPoor: Opened with a Key
There is a Mirror in Downpoor 1/2 were you have to Touch the white Fire, wich is randomly placed on that floor and then enter the Mirror.
You will be turned into the lost. Dying now will end your run. (Leaving the mirror will turn you back)
Now you are in an Mirrorworld were you have to repeat the rooms and get to the item room for Knife Part 1
You can kill the Boss for an additional Boss Pool item but it is Risky.

Mines: Bomb the door twice
In the Mines 1/2 there is a Minecartroom.
There are multiple yellow buttons on that floor wich you have to press wich opens the door in the Minecartroom.
Get to the End of the labyrinth. There will be Knife Part 2 on a pedastal but now you have to run away and play temple run (flying will make this section A LOT easyer)(flying maybe gets disabled here? Only Characters with inate Flying like Lost/Azael keep it? Needs testing)

The Mousoleum: Pay 2 Hearts to enter
In The Mousoleum, you have to kill the special mom version wich spawn the womb door with an X on it Shoot with the Knife against it, it works like bobs rotton brain, then you have to kill Moms heart. (after that you can't go back so complete boss rush/shops before

Now everything will turn gray and you can enter The Corpse Complete this Act as usal just like the Womb.
The boss on the 2. Floor here will be Moms </3 / Moms broken heart? kill this boss and you will unlock the new character.