Comment by r_lee

yes,

in short, people/entities with websites employ bots to click and interact with the Google Ads on their website to generate fake ad revenue.

and the person running the ad campaign is billed for those fake interactions.

I'm guessing app ads are heavily botted because they're more costly vs. a simple ad that lands you on a website (edit: I was wrong, see reply by OP: https://news.ycombinator.com/item?id=49665323)