Advertisers can reserve ads on certain channels by going through the sales team, however, this only applies if the advertiser wants to spend a lot of money. Like, "L'Oreal wanting all ads on beauty channels for a week"-kind of money.
Advertisers can also just do placement targeting onto a single channel, however, there's no guarantee that their ad will be played in front of the video - other advertisers can out-bid them, even if those other advertisers don't care about your channel in particular.
Because of this, what usually happens is that instead of doing pre-rolls, advertisers interested in closer relationships with creators are doing letting creators do endorsements instead. These involve the creator making the endorsement themselves (usually by following a script the advertiser gave them) near the end of the video, and a very quick "this video was sponsored by " near the beginning of the video.
Note that there are rules on what you can and cannot do for these endorsements: https://support.google.com/youtube/answer/154235?hl=en
Most importantly, you mustn't burn pre-rolls into your video.