I've worked for a few companies that had ad placements. I wasn't too deep into that side of things, and it was a long time ago, but as I recall, at reddit there was an in house ad auction platform. If there wasn't any ads sold for the period, we'd either show in house ads (think the old reddit merch store, pics of animals, a pic of one of the reddit staff with a paper tube on his forehead to resemble a narwhal, etc) or ads from a network like AdSense. Once upon a time this actually caused issues because there was malware being served from one of those and networks