I can buy ads generically on a site, but what happens after the user clicks on the ad or just views the ad and then buys the product? How do you keep track of t

I can buy ads generically on a site, but what happens after the user clicks on the ad or just views the ad and then buys the product? How do you keep track of this when you can't measure the effect? The only way is to measure some form of before/after analysis of your conversions with or without the marketing campaigns... Too much noise to be able to do that on the web.