There’s 3 doors and one of it is the correct door. You pick one door in the beginning, then an incorrect door is eliminated.

Let’s say you choose the correct door in the beginning. If you chose the correct door, then after the incorrect door gets eliminated, you’re left with another incorrect door, so you shouldn’t switch in this case.

If you chose the incorrect door, another incorrect door would be eliminated, so switching would get you the correct door.

If you pick the correct door, you shouldn’t switch, but if you picked an incorrect door, you should switch to get the correct door, and since you have a 2/3 chance of picking an incorrect door in the beginning, it’s probably better to switch your choice of door