Ad is inserted into stream. Either one of two things happens depending upon the way it is implemented:
The length of the video stream increases as the ad is inserted suddenly. The ad blocker can simply calculate the difference and skip the difference worth of time, thus skipping the ad.
The length of the video doesn’t increase to prevent this. Thus, you get the ad stream overlapping in front of the actual video stream. This would thus kinda be on the frontend, which could easily be blocked.
The ad is inserted in the beginning itself at some random time in the video. Hence, the length of the video doesn’t change suddenly like in scenario one. However, remember that regulations require you to visually indicate that a given piece of media is an ad or not. This is why YouTube ads have “Ad” in a yellow box. This could thus very easily be detected by an adblocker that analyses every frame that the box is present in, and skips that frame. This however, would be a little heavier for the user using the adblocker.
Trust me lol. There is literally no way you can prevent ad blockers.