system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How can people upload premium adult videos without the DMCA busting them? [NSFW]

Example hdpornbase.com, premium stuff, no sponsor nothing, and obviously they are making money off of ads, with content they clearly do not own.
gpt
The video content is actually supplied by the producers/owners of that material to their affiliates and "tube" site owners as a method of marketing.

Imagine if a new porn star arrived on the scene, and the company that produced her content added clips of her work to the popular adult tube sites. Visitors would find the clips, a percentage of them will really like what they see, and either click through directly from the tube site (which makes the tube site $$$ in affiliate revenue) or Google that porn actress; either way, they end up on the producers website ready to sign up and make the company money.