It’s not literally part of the video, exactly because of what you describe. They are separate streams that get injected into the player before the normal video. You can’t skip them or interact with them in any way (pretty sure it also breaks any purchase links etc). Piped or Invidious don’t have them, ytdl also doesn’t download them.

As of now, afaik, you won’t see them if your account wasn’t selected for the experiment, if you are in incognito mode (with uBO on) or if you have uBlock Origin (and other adblockers) off (you’ll see the normal ads and then the video).

Otherwise, apply uBO new script if you get them