This is a question about YouTube rather than a question about the TubeBuddy plugin specificially, so I'm moving this to YouTube Discussion.
My observation of my own channel shows that if your video matches what your viewer is watching in terms of topic (title) and niche (gaming, art etc) and it has been watched for a good amount of time (less dependant on views and more dependant of average view duration) then a video is more likely to get suggested. It's not guaranteed, but that's what I'm noticing.
For example, I have a few spinosaurus drawing tutorials. If someone watches a documentory on spinosaurus, and they have a history of watching art... they MIGHT get my higher performing spinosaurus drawing tutorials suggested to them in the side bar of the video they're watching, or in the end card, because YouTube's algorithm, based on the viewer's watching history, knows this person is watching A spinosaurus video, so logically, they MIGHT want to watch a spinosaurus drawing video.
Another example. I was watching a Nick Nimmin live stream over the weekend. Because Roberto Blake is also in the same space as Nick Nimmin, youtube suggested some of his videos too, because based on my viewing history, they determined I MIGHT watch those videos. And it's the videos that have been watched the longest, that logicially might be watched even more, so it's those videos the algorithm suggests.
Pre 2018 the suggested algorithm seemed to follow title and views only. Now days it's heavily reliant on audience retention. My videos average 10% audience retention meaning only 10% watch to the end, meaning they're less likely to get suggested
The green line shows my suggested traffic. and in 2018 the algorithm started following how long the video was watched for, not just the title.
This video has pretty good average view duration. not the best, but better than my average
As a result, my traffic comes a lot from suggested videos. One of my most suggested videos.
Meanwhile this video has 6.2% which is pretty horrible.
as a result, suggested impressions don't even show up in my traffic sources at all.
So with these factors in mind, it's pretty obvious that how much of your video was watched, will determing if it gets suggested more than other videos. And of course... A video about drawing dinosaurs isn't going to get recommended on a video of someone doing drum covers, so title and niche is important as well. And your viewing behaviour means if you don't watch art videos, it's highly unlikely you'll ever be recommended one of my videos.
Hope that explains it a bit more, it's something I'm currently studying because I'm trying to boost my own suggested/recommended traffic at the moment.