it won't though, because you can just remove that 30 seconds at the beginning, which is almost definitely going to be very different than the rest of the video in a number of ways. Notably, there are likely going to be UI differences during and after ads play, as well as video playback alterations. Ad's aren't going to be the same quality as video itself.
It's possible that they're transcoding them into the video itself, but doing that would be catastrophically bad and have such a massive cost that it simply would not be worthwhile.
They are transcoding them into the video. Sponsorblock had to make a quick change to discard submissions from users that have been identified to be on this trial system, because it affects the video length, and as such - makes it impossible to have consistent segments
i highly doubt it. I would think they're probably doing some UDP packet voodoo bullshit.
Though it likely appears as transcoded.
The sheer cost of them being transcoded into videos is immense, even if they're live encoding every video.
What happens when you get an ad you need to takedown and remove? You're on disk transcode is suddenly useless now, and you need to make a new one, easy enough, you can just do that in the background, but this also means your ads are baked into each video, which is less than ideal, unless you're constantly updating them.
And if you're doing live transcodes, that means that you have to do this for every view on every video, and i'm not sure that's sustainable.
I suppose you could probably do a cached live transcode system to bring down the overhead, but i can't imagine it's easier than just pulling some voodoo networking bullshit to literally inject an advertisement.
AFAIK there is no need to re-encode, since Youtube videos are stored and served in chunks anyways. The change is that they are now slipping in the ad chunks as if they were a part of the normal video chunk stream.