"Quality" is VERY subjective and depends on context, niche, and intention of the viewer.
I have seen many videos that were not of "high quality" in terms of production value but did very well because it delivered the info I wanted. Sometimes the best gems are the "unpolished" and "real" videos.
I know many people who believe 4K is the only way to go for "high quality". That is true if you are looking at "video quality" only. It may or may not be true in some situations.
There is one 2 million subscriber Youtube expert who continues to increase the production value to the point it is becoming artsy and cinematic. I've begun to lose interest because he is losing the relatability he once had. Having said, that he will do just fine without my viewership.
If I want high production values, I tend to watch TV, movies, or documentaries. On Youtube, I don't seek that kind of production value. I seek more grounded material that come from "real people".
Having said all that, it is impossible to define "high quality" specifically. It contains many different variables depending on the subject matter, context, the intended audience, and purpose of the video.