For the past few months, I've been using YouTube's "suggest placements" feature to put ads on my videos. Sometimes, it decides to put an ad in the middle of a video. Other times, it wants two ads per video: one at the start and one at the end, but they're always picture ads, not video ads. Or at least, that's what I've seen on other channels!
I have no idea what the criteria is, for which type of ad you get. If YouTube suggests the two ad setup, and one is in the first thirty seconds, I always decline it, because I find that super annoying when I watch videos myself.
I've also used YouTube's endscreens feature. You know, where a subscribe button, a link to my Patreon account and a recommended video pop up, for people to click on. I did this for months, before being told that the endscreens cover up the entire video, so viewers can't see anything. That's annoying! I thought people could minimize them / turn them off. So I had to go back through dozens of videos and manually turn off the endscreens. From now on, I'm going to try leaving a 20 second black screen at the end of each video, and put the endscreen there. That way, it's black screen that gets covered, and not an actual video.
I've been told the "official" way to use endscreens is to have your video running in the upper/left corner, while the endscreens cover up every other part of the screen. The idea is that people will still watch, because the video is technically still going. People are more likely to click out of a video right away, and not explore the endscreen links, if it's just a black screen. I'm not doing this for two reasons, though. #1, that involves editing prowess that I don't have. #2, I don't like it when other YouTubers purposely shrink a video to make room for ads. If someone doesn't want to watch my endscreens, I'm not going to try to trick them into doing so.
The endscreen video also said to never make a complete video. You want to end each video on a cliffhanger, so viewers will be hungry for more content and click on the link to the next video. Why, they will be grateful to you for providing that link! I say "not true!" Viewers will be mad you wasted their time with an incomplete video. I'm pretty sure viewers get mad if you add fluff to your video, such as an extremely long intro, just to make it longer. So I try to avoid that, too; I generally do intros every other video, unless it's something like Phoenix Wright where I do an intro every video, just so I know where to cut off videos. (Editing those videos is tough; the intro is my cheater way of making the editing process easier)
Just in general, my strategy is to avoid doing things on YouTube which I find annoying, because I bet my viewers will find them annoying, too.