It's mostly a matter of disciplined tracking and a decent chunk of boring work getting all the systems integrated. You click on a Target ad, go to their website

It's mostly a matter of disciplined tracking and a decent chunk of boring work getting all the systems integrated. You click on a Target ad, go to their website (which you are logged into and have cookies), and get some attribution flags set on your account. When you make a purchase online (easy) or in-store (less easy, likely needs a rewards account linked), you credit that ad/campaign/channel with the conversion value. It's not a perfect measure of the lift, but it's pretty solid and underestimates things if anything, which makes it good enough for basing ad buying decisions off of. You could do fancier geographic segmenting + statistics, but why bother when the direct method works? It's actually easier for large brands, since they can afford to put engineering effort into building bespoke tracking and analysis software. The trickiest would be smaller brands, especially those who advertise via agencies - they can't do anything that isn't commercial off-the-shelf, and as of now there isn't anything great in this space.