They get some control but it depends on the content how exactly it works. I think for normal videos they get a say right from the start where the breaks are. But I know one guy who does YT and he live streams and has to clean up every VOD because they just randomly pepper ads all throughout.

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.