Yes I finally understood it. At the beginning you are choosing one door vs "the others". If you could you would already chose "the others" because they have a 6

Yes I finally understood it. At the beginning you are choosing one door vs "the others". If you could you would already chose "the others" because they have a 67% win probability but you are only allowed to chose one single door. Then he opens all the wrong doors of "the other doors". The "other doors" still have 67% probability and now only one door is remaining in the "other doors". Obviously that last remaining door now has 67%.