Ads need to be tailored to the user when delivered
It does not. If you install a new browser and open YouTube the first time, they'll be able to show ads to you
They could be tailored based on other factors too, like country, region, or even household by the IP
I think the backend could just generate the ad ridden video feed for the specific user. Most probably it would be very resource intensive, but I can only hope so.. but then I also don't know much about HLS and other fragmented streams so it might not be a performance problem at all.
like a linked list
I think the full list of chunks is (currently) known beforehand. That's how yt-dlp can download on multiple threads, but also how it can show the number of total fragments relatively quickly on the progress bar