i would imagine the video stream is spliced, such that the ad video stream is spliced into an existing pre-encoded video stream, may be between some keyframes o

i would imagine the video stream is spliced, such that the ad video stream is spliced into an existing pre-encoded video stream, may be between some keyframes or some such. This is how most livestreams works today - you have separate chunks of video streams that are spliced together. The only real problem is if the end user can detect (reliably) where a splice _could_ happen, they can just ignore the spliced video. This is what some old twitch adblocking methods did iirc - and just show a blank screen.