system
Given the following post, respond as if you're commenting on Reddit:
human
Eli5- Why do adds for mainstream/big name companies exist on random (illegitimate) streaming etc websites online? How does the revenue stream work?
gpt
Basically the big company posts an ad on an ad company's server, then the streaming website signs up with the ad company to earn money showing ads to its users.  The ad company server records how many people see / click on the ad, and which website they saw it on.

Hundreds, maybe thousands of sites sign up with the ad company.  The ad company uses the records on its server to figure out the financial implications:  The ad company bills the big company $1 for every person who clicked on the ad, or every 1000 people who saw the ad.  Then the ad company pays $0.25 of that $1 to the streaming site and keeps the remaining $0.75.

Every site that shows ads makes money for the ad company, so it has an incentive not to investigate too closely whether the sites that have signed up are showing pirated content or other illegal activity.

This is leaving out the possibility of straight-up fraud.  For example the ad company could lie to the big company about how many people clicked / saw the ad.  Or a website operator could rent a bunch of computers to run scripts to automatically watch videos on their site all day.  Or the website pay a roomful of people $0.10 per ad to click on ads all day.  This kind of activity is of course highly unethical, probably a major contract violation that means the ad company / website could be sued for whatever they made plus lawyer fees and punitive damages, and it may be illegal enough to send the ad company / website's owners / top managers to jail.

But none of that stops shady ad companies / websites from trying to make money this way.  The sort of shady people who do this kind of thing know how to try to hide their activities by working through shell companies in various countries and so on.  The big companies buying ads and the legitimate ad companies try to figure out how to detect "fake" clicks / views, and the shady website operators try to figure out how to make fake clicks look real enough to fool the fake click detectors.  It's a game of cat and mouse.