They use ad network to try to target individual user visiting a page (eg you know your target is a male, 60 years old, living in that area, with a few other checks you can almost uniquely identify your target)
Then the ad network delivers a targeted ad that exploits a vulnerability (eg a broken JPEG decoder that, when sent a bad image, allows to run arbitrary code on your machine), and from there you have access to the computer
Then the ad network delivers a targeted ad that exploits a vulnerability (eg a broken JPEG decoder that, when sent a bad image, allows to run arbitrary code on your machine), and from there you have access to the computer