it's automatic and it's determined by YouTubes algorithm and also the ads that are being sold at that given time.
So if you have a video on lets say, steemit, if steemit was running ads on youtube or maybe one of steemits competitors were aswell , youtube would know that those ads defintely belong on your video and you will get paid more for that, then you would for let's say a life insurance commercial. Just because it is relevant to your content and your viewers eyes are worth more to that company then others.