What are the objective mechanics that determine Google’s evaluation of ‘relevance’, ‘quality’ and ‘depth’

So I'm a full stack web dev (and, in another life long ago, a technical writer) but haven't had to work on SEO until recently, and I'm really struggling with this. I understand some of the very basic measures of engagement that partially determine your ranking on Google and are objectively quantifiable–number of clicks, time spent on page, number of links, etc. What I don't get is metrics like ‘quality’, ‘relevance’ and ‘depth’. These seem highly subjective. Relevant to who? If it’s ‘relevance to users making queries on subject X’, as my buddy ChatGPT tells me, well that doesn’t get you too far when you can’t poll the userbase–how do you know whether, say, an article about broad trends in the field of ecology is more relevant and will rank higher than an analysis of an important individual ecological study that just came out? Which puts you at the tip-top of the search results, which makes you appear on page 2?

With ‘depth’ I feel it gets worse. Deep according to whom, and in what sense? Are we talking summary or analysis? Is a paragraph making a revolutionary observation about a field of inquiry that doesn’t do a lot of summarizing of the basics of that field ‘deeper’ in SEO terms than a long, exhaustive, Wikipedia-ish summary of everything about that field that makes no innovative observations whatsoever, or does Google like the latter better?

And, as you might guess, ‘quality’ is the most confusing of all to me. What are the criteria making one piece of content of better quality than another, once you get past very minimal requirements like grammatical correctness and formatting? Say Blogger X makes edgy, abrasive commentary on a topic using short, punchy sentences, without lingering too much on the nuances of a topic; Blogger Y uses longer sentences that prioritize subtlety and thoughtfulness. Some people might say Blogger X has the higher quality content, because it’s more entertaining and its shock value will draw in a broader number of readers; others might find that stuff irrelevant, and consider Blogger Y to be superior, because his content has greater depth to it, which might draw in a readership with less breadth, but more likely to consistently return to the site?

Can any of you SEO whizzes tell me what's going underneath the hood with the quality/relevance/depth metrics? I guess that's kind of what I'm looking for. What are the objective mechanics of the process Google uses to evaluate them? And if there aren’t any, how the heck do you figure out how to ‘optimize’ your content for them? Because right now I feel like I feel like I'm trying to satisfy both a literary critic and ace a code review and the two processes don't mesh well in my brain.