For me, in more recent months it's been about understanding the relationship between impressions and CTR. I'm an artist so "Data" has always been like maths to me, it takes me a long time to understand, and for the longest time I thought having a 5 or 6% CTR was amazing, but as
@Andrew pointed out more and more this year in tubebuddy streams, that gradually made more and more sense, is that impressions is super important, because 5% of 100 impressions isn't very good, it means only 100 people have been shown the thumbnail. So it's important to get the impressions up. More impressions leads to more chances to get views, then once your impressions are in the thousands, THEN the ctr percentage becomes more important and gives a better indication of the performance of your video and thumbnail as a first impression.
my "oh!" moment of 2019 was the importance of using longer term keywords to rank better. It's near impossible to rank number 1, for "dinosaurs" seems "Dinosaurs" encompass dinosaur art, dinosaur fossils, dinosaur games, dinosaur movies, dinosaur comedy... and just like "art" encompasses dinosaur art, sculpting art, drawing art, photographic art, traditional painting... millions upon millions of different videos fall under "Art" so using a single word tag is kind of pointless on youtube. "drawing a dilophosaurus head" is far more specific and far more likely to get me suggested as a search result... and now after changing my mindset, I'm ranking in more areas.
Also the importance of using suggested videos in your own descriptions, pinned comments and suggested videos at drop off points in videos.