I assume you’d have a ‘main stack’ as in a bigger pipe that runs up the whole building, and then the smaller pipes that would branch out to toilets, showers, etc from there. So once the waste got to main stack then yes it would just fall all the way down.
As for the other question, there are usually access points to reach pipes. So once a plumber determined the location of the clog they’d just fix it from the nearest available one.
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