Do you mean that creators choose explicitly how many ads to insert and their length? Because if it's just an ON/OFF switch and then Youtube proceeds to put almost equal lengths of ads than of content (1+ min ad after 2 minutes of content, as per the previous comment) then that's truly terrible.

Also... ads after only 2 minutes? Am I the only one who thinks that feels much worse to bear than longer ads more spaced out? There's something that feels odd with having to get interrupted by an ad after mere 100 seconds of watching something.

Yes. They can either let YouTube stick ads in or they can manually place them if they opt into mid-roll ads.

This is a bit misleading, IIUC. "Content creator" in this case means the nominal copyright-holder, which often means someone simply claiming copyright, in practice, and not necessarily the uploader ("Fair Use" be damned). The person who uploaded the video might have fully intended there to be no ads run during playback, but has little say in the matter if someone comes along and claims that their content is used in the video.