
I just don’t get this dom/sub shit or something like the BDSM trope they are getting on. I mean, no offence but is it really a thing? Like, outside the porn industry? And why do people even like it? Is it something worth the pain and the exhaustion and whatever?

Agreed. It’s internalized homophobia with dom kinks and I’m just not about it. I would like a more healthy representation of bdsm where it’s just two partners trusting each other and exploring this with each other. But I guess that doesn’t make for ~good writing~ but I’m getting tired of these tropes too.

The way bls do it make it look extremely toxic and abusive. In real life, it’s 100% consensual and you sign a contract and get on the same page before you even do anything. In bls they make it seem like the dom has control of the relationship when in reality it’s more of the sub that controls things.They make a safe word that may be something like “flower” and as soon as someone says that word they stop no matter what. I’m not really good at explaining things so I hope that makes sense.

BDSM is a fairly common sexual ... pastime(?). Idk the word I’m looking for but it’s fairly common in porn and in real life. Most depictions of it in media can be exaggerated but the main idea is both parties are consenting to enter into a sexual play for both their pleasure and to do things they both agree too. Consent and mutual pleasure are a big thing in the community. It’s really a way for people to safely and consensually to explore their kinks.

Yes, it's really a thing! For a lot of people, the pain is part of the fun. It's a fetish, so it's not everyone's cup of tea, but it's fun for people who are into it. If you want to read a manga that explains it REALLY well (including aftercare and stuff), I'd definitely recommend Nana to Kaoru. There's also a comic that explains it really well with a lesbian couple as the main characters called Sunstone; the artist posted Sunstone for free on deviantart and a few other places.
Why is this “concubine” thing so popular in ancient Chinese manga set? Is it like a real thing in ancient China? Somewhat like kings having multiple affairs as and the likes?
Yes its a real thing. It wasn't just China either. Many kings did it.
And if it wasn't a concubine, many would at least remarry at some point
But, um, did only China have this somewhat glorified concubine tradition which they were given palaces of a kingdom and several personal servants? I have no memory of any other emperors or kings being this “generous” to their concubines (except for Solomon with his tremendous amounts of wives like lol did his dick not fall off? And I do not also recall him give “palaces” and personal servants to these wives).
Can't really help there. I have always been more interested in mythology than the actual history, so I don't know much about the subject.
I think some of this is embellished to make the story plot more interesting, bc a lot of book and stories I’ve read (not specifically Chinese) has had all the concubines living in the same palace; but maybe in China ur functioned a little differently.