Hopefully somebody who actually makes monetized Youtube videos will join the conversation to answer that one, as I'm not certain. I'm a pretty active Youtube watcher and fairly savvy on the culture, so from what I've gleaned I believe there is some control given to creators but I believe it is somewhat limited. For example if you watch the Sorted channel (a UK-based food channel) with ads on, they seem to pretty consistently happen at small scene transitions, which leads me to believe the Sorted team is doing their part to strategically place them.