> Is there a measurable effect? What kind of businesses see uptick in sales, and how real / scammy they are?
There are plenty of mismatched incentives in the advertising supply chain making it so that very few of the parties involved actually care about an uptick in sales.
You have the marketing department in the company, who gets allocated a certain budget. Their objective is to spend the budget (otherwise they'd get less next time) and find a way to convince their boss that it led to good results so they can justify their own salary/promotion/etc. Sales is a difficult thing to obtain (let alone attribute), but there's this magical thing called "engagement". You can pull it out of thin air.
The marketing/ad agency they hired needs to justify their fee. Again, sales are a difficult thing to obtain (for a start, the product needs to be good and useful to the purchaser) let alone attribute. But again, your objective isn't really sales when you can just show "engagement" and let someone else find an excuse as to how this relates to sales, if any.
The advertising platform is happy to take their money and send (or outright make up) "engagement" their way. They don't care how real or intentional this engagement is. Someone trying to dismiss an obnoxious cookie banner and clicking the ad by mistake? Great! Obnoxious lockscreen ad and person thinks they have to tap the ad to unlock the phone (this is real - I've seen it)? Sure thing!
The websites in turn are paid per click they refer to the advertising platform and don't care about how real/intentional those clicks are either, so placing the ads in inconvenient places where they're likely to attract unintentional clicks is in their interest.
At every layer someone skims some money off the top so everyone is happy to look the other way, and nobody is interested in exploring other approaches (such as a marketplace model where users who intend to buy something can search through ads) because such a model, even if it turns out more efficient in connecting sellers & customers, will eliminate the need for these middlemen which is an obvious problem for them.