"parts of the main video that are ads" - well, i'm not sure what those are. Is it something that happens when they record a program from a TV and then upload that video to youtube without removing TV ads?
Imagine a Youtube video where the host takes a break to read off an advertisement and that you'll get a 10% discount if you use their code during the checkout process. And then they resume with their content.
They often will pre-record these segments with a consistent "now with a quick word about our sponsors" jingle which can be statically detected and removed from the video.
It means things like sponsor spots where the YouTuber starts talking about some random mobile game or VPN provider you can tell they don't really care about