if you’re downloading the video locally it would be incredibly trivial to remove the segment of the ad. There are various different mechanisms i can think of that would work.
obviously, beginning and end ads are super trivial.
Ideally, youtube won’t be natively encoding the ads into the videos, because that would be a nightmare, so presumably they’re doing injection instead, that would be pretty obvious from the get go.
If not, they have to have some kind of interface for the advertisement you could very easily use that to track the ad placement itself, though that might be problematic.
There are likely other clever things that can be done, we’ll have to see what happens.