I've been watching High&Low on Netflix. I highly recommend the films - they're highly stylized but the action is surprisingly diverse and exciting. Parkour, kickboxing, some MMA, and the plots and acting are much better than you'd think (but not good, don't get me wrong). Also lovely music and great, great Japanese street fashion, if you care (I am not fashionable but I enjoy a good outfit on screen).
I've also read a number of gangbanger manga and manhua. I don't know if there's a better name for the genre - stories about kids in rough high schools who belong to gangs and settle things with their firsts (and feet). Some have really good stories and characters, some are funny - it's a range.
What's interesting to me is that the stuntmen and stunt coordinators for these films are clearly martial artists and informed by martial arts. The characters throw good boxing combinations, kicks with solid technique, etc. The fights look like fights from a streetfight in an MMA movie - it's not just two guys standing at melee range and throwing haymakers until one falls down (though that might happen occasionally, it's rare).
Since the actors are using martial arts, and those martial arts are a key to the plot, why would I say these are NOT martial arts movies?
To me, a martial art is a system of movements that have martial intent. The system can be loose - I don't mean to say a martial art must have a single response to any particular attack - but it has to have moves (punches, kicks, throws, joint locks) that make sense as a response to an attack (someone else punching, kicking, etc.). There has to be a 'right' way to do things. You can have different jabs in your boxing system, but there are also movements of the arm that don't count as jabs. And this system has to be taught, it has to be teachable and trainable. Otherwise it's not an art.
In a 'real' martial arts movie the characters have to be martial artists. There has to at least be the implication that they trained - that they were taught a system, and are implementing a system, in combat. It's fine if they are using a system that they changed themselves, but it has to be a system, not just "being really naturally good at fighting." And I'm fine with films where the training isn't explored on camera. Some films have training montages, and some do not, but in true martial arts films it is at least implied that the characters are heavily trained.
In the gangbanger manhua and films there is very little implication of training. There are no or very few montages. The best fighters are just naturally better fighters - they're inhumanly athletic, but you never see them train or practice fighting (again, with rare exceptions). There is some sense that the morally just characters are also better at fighting, or at least the ones who are more emotionally committed. But again, no training, no system, at least in the majority of cases.
In High & Low the Amamiya brothers are shown being trained as kids, but EVERY OTHER character get tough just by fighting a lot. In their down time they're just smoking and hanging out with friends, not practicing their jumping kicks or doing pushups. I think there is one other scene where a character is doing pullups when approached.
This doesn't make the movies bad. They just aren't promoting martial arts virtues - the commitment to discipline, to training, to self improvement. Those aren't messages in these movies. The tough are tough by birth, and the weak are... weak. If anything, these films use martial arts movements to give a sort of anti-martial arts message.
If I were able to make a high school fighting movie I'd love for a group of friends to get their asses kicked early, find a down on his luck instructor (probably someone's older brother) to teach them, train ridiculously hard using makeshift implements, then go out in a triumphant battle. That would be a martial arts movie. An even truer martial arts movie would have the friends decide they didn't want to be involved in streetfighting at all, and go off to college or something at the end, secure that they could fight if they had to. But High&Low isn't any of that.
Just to be perfectly clear, this isn't a criticism. High&Low isn't billed as a martial arts movie, it's not pretending to be one. Just food for thought.