Sometimes a DM may create a new setting for a game, sometimes they don't. Most people only play a campaign for a year or so, that's plenty of time to create a setting even for people new to the game (I created the basis for my campaign world after playing for a year or so). Maybe it's a setting the DM has been noodling about for years, maybe it's inspired by a favorite set of novels or movies, maybe it's something they came up with after eating too many burritos. It's still the DM's creation and I'm going to abide by their final decision, and no they don't have to explain it.
It's also irrelevant. So keep this simple and answer a related question you've never answered. A DM says no evil PCs. The DM is the one running the game, there really is no compromise here. You want to play an evil PC. What happens?
My guess, you talk about it.
Why does the GM not want evil PCs? What does evil mean in his campaign?
Why does the player want an evil PC? How does he think will it affect the campaign?
Say, the GM says he doesn't want evil PCs because they want the characters to be motivated into action simply because it's the right thing to do.
The player says he wants his character to ignore people in need, burn he poor, steal from the party and murder anyone that objects, eat the occassional child and maybe even steal a lolly from a baby.
That might
genuinely not work out for the campaign. Maybe it's really time to shelve the idea for another time, or go seperate ways.
Maybe the player says he wants to play a criminal that did bad stuff, and he believes he must be evil to justify this back story. But he doesn't need the character to actually still be doing all that.
So maybe the compromise is that the character maybe was evil once, or just neutral and thought his actions were acceptable or necessary to the manipulation of others. But he realized it wasn't okay and is now trying to atone.
Maybe the player wants to be a necromancer, summoning the dead, and that is by RAW evil. Maybe it doesn't have to be in this campaign, maybe there is some deal with the death goddess that some necromancers have that allows doing so without harming souls or the cosmic order, but it might come at a prize later.
Or maybe he doesn't need it to be necromancy, maybe he just likes some mechanical features like being surrounded by minions, and a reskinning of the spells into summoning spells or creating shadow illusions from corpses.
Maybe the player character used to be a necromancer but realized he harmed people, and tries something new - but he still got the necromancer specialization until he can retrain it with a good story explanation (or because whatever the game's necromancy feature is, keeps it because it doesn't really involve creating undead)
The thing is, peole like to state that this DM/player conflict is an immovable object against unstoppable force. But likely the extremes aren't as hard, and there is a way to find common ground for a compromise than can work for both. It's almost never that extreme..