Yeah I'm pretty sure that Youtubers can only pick where the advert goes. There's a few who I follow where they edit their videos to designate a cut where the advert would go in the middle.
Then there are the ones who go to ridiculous lengths - like there was some youtube drama and this person 'telling their side of the story' had a one hour-ish video that was straight up RIDDLED with adverts every few minutes. It was only watchable on my laptop where I have adblocker; but looking at it on my tablet the advert breaks marked in the videos was just downright silly.