Launch HNs for YC startups get placed on the front page. This is in the FAQ: https://news.ycombinator.com/newsfaq.html.

The instructions for YC founders are here: https://news.ycombinator.com/yli.html, if anyone wants to take a look.

I think most people here consider it fair that HN gives certain things back to YC in exchange for funding it.

Ideally these posts should have some sort of "promoted" tag next to them.