It's a mixed bag. Some ads (like some Youtube stuff I guess) are bundled and filtered, but most actually rely on external requests to ad exchanges. What happens mostly is that when there is an ad spot in the page you downloaded, that is in fact a generic request to an ad broker to send an ad instead of a specific ad. That then starts a real time bidding process inside multiple broker networks to find the most expensive (for the advertiser) ad they can show you based on your tracking information and demographics.

And that's for every ad spot. It's insanely intricate and frankly wasteful.