It's a language so that people can discuss varying levels of nuance with each other. You probably don't care about the difference between industrial metal and folk metal, but metalheads certainly do. And even that's a pretty top level example.

Same goes for every other genre/subgenre. Indie pop and bubble pop can be very different, and just because someone likes one doesn't mean they'll like the other.

There is also a historic factor in all this because genres are always influenced, derived from, or mixed with other genres. Triphop was born this way, as a mix of hiphop, jazz, dub, electronica, and many others

Tldr: Think of it as painters describing colors

I don't think anyone understands ALL of them.

As you become familiar with a certain style, you notice differences between them. The deeper you dive and the more you listen to, the finer details you can recognize.

I could probably name at least 25 different genres of rock, and explain how they are different, but styles I'm not familiar with, I can usually only recognize the broadest category. Bluegrass all sounds like bluegrass to me, but an expert could identify 25 different subgenres of it.

I've actually been looking into how music genres/subgenres are defined for the past few months due to the fact that my favorite genre "doesn't exist" (I'm not joking someone wrote a research paper on this)

I think there are research articles on this if one wants to go into details... Like how certain genres separate. Sometimes there are strict definitions (most techno I think are quite well-defined). But practically I think most ppl tend to enjoy ranges of genres that are close to each other... There are also plenty of genre-blend songs too so there's that

Also I second for Every Noise At Once, they have some really obscure genres too for detailed comparisons

It’s not very intuitive, for sure. It really is about if you were there at the time and probably place (up until, say, 2005) hearing XYZ song/style becoming popular on the radio and mass media. Only then do you pick up the lingo and see how other people referred to it.

Plus, go browse the racks at a music store in another country (preferably where the main language is different from your own). I remember seeing a Tower Records in a major Japanese city have a whole area designated as “Black Music” (not like a seasonal feature/display, but a section/‘genre’) and being baffled, like all soul blues funk R&B early rap gangsta rap and dance pop was all one section (filtered only by skin color)? But then again, is all J-pop or K-pop always the same style, or do English-language listeners lump a lot of things together by overarching basic criteria? (Like just shuffling music off based on country/language of origin etc)

Alternative rock was different to the popular rock from on mainstream stations in the 70s/80s and early 90s. See also “college rock.” But then alternative rock becomes mainstream and fractures into subgenres (for example, compare Limp Bizmit once called “nü rock” and White Stripes “garage rock” — fairly different styles), less mainstream artists continue plugging away in their various “indie” lanes, plus what does “college rock” (like REM of the 80s) mean in the 2010s/2020s anyway? Short answer: big fat nothing; long answer is, first you start doing deep dives in music journalism of yore, browsing the categories/sections at record stores, talk to old music heads and hear their opinions (mostly opinions, right, less ‘facts’), and then you have a better lay of the many various musical landscapes and keep exploring.

Does it play on the oldies station? Then it's an oldie.

Does it play on the rock station? Then it's rock.

Does it play on the pop station, and sound like the most manufactured, fake, and passionless bullshit you ever heard? Then it is most definitely pop music.

This is what happens when you pay too much attention to any kind of taxonomy. Even scientific and rigorous taxonomy disintegrates into subjectivity when you look too close.

The idea of precisely labeling and categorizing things appears to be a human desire imposed upon an uncooperative universe.