Monetisation & Revenue
Ad Inventory
Also called: inventory, available impressions
Every place an ad can run is a unit of inventory: the pre-roll, mid-roll and other formats attached to a monetised video, plus the ads served between Shorts in the feed. Advertisers bid for it through an auction, so the price is set by how many of them want that audience against how many slots exist.
Inventory is why earnings swing on a calendar rather than on anything a creator did. Rates run higher in the fourth quarter, December especially, because advertisers are spending down annual budgets, and the new year resets that while the supply of videos stays roughly flat. Nothing about the channel changed.