You have three doors
One of the doors is a winner (car)
Each original door has a 33% or 1/3 chance of being the winning door.
So if you choose two doors then your chances of winning are 66% or 2/3.
If one of your doors are revealed and it’s not the winning door, in theory your odds are still the same. You still have a 66% chance of winning the car from the original circumstances. So in reverse if you choose one door you have a 33% or 1/3 chance of winning and the other two doors have a 66% or 2/3 chance of winning. By revealing one of the other doors, you still have a 66% chance of winning with the remaining door, so changing to that door gives you a probability of 66% or 2/3 chance of winning.
BTW if you have two doors, your chances are 50/50
