What's Fate actually about?

It's about the Holy Grail War. The grail is an omnipotent wish-granting device. Mages seek the grail for ultimate power.
The reason for all the spinoffs stems from the original Visual Novels, they had 3 routes you could play through- it was originally an eroge novel with Shirou Emiya as the protagonist. So the first route has Arturia (Saber) as the love interest, 2nd has Tosaka Rin as the love interest, and 3rd had Sakura Matou as love interest. All 3 routes were adapted to anime. Then they created FGO as an mmorpg and there were various routes in that, so even more spinoffs came out. Basically it turned into a multiverse. I don't get why it's hated so much, I assume it's because the lore is dense, and things that are difficult to understand people find intimidating.

Such bizarre questions. Imagine asking the same about Batman?
"Why do they mostly make spin-offs of different Batmen? Why are there usually different characters as protagonists in respective Batman spin-offs? Why are most of the source material of Batman are written by different people?"
In the interest of being helpful, nevertheless, I will do my best to answer your questions.


Money.


They use whatever characters fit the stories they wish to tell.


Because the original writer does not wish to spend his limited time writing about magical girls and whatnot.

Such bizarre questions. Imagine asking the same about Batman?
"Why do they mostly make spin-offs of different Batmen? Why are there usually different characters as protagonists in respective Batman spin-offs? Why are most of the source material of Batman are written by different people?"
In the interest of being helpful, nevertheless, I will do my best to answer your questions.


Money.


They use whatever characters fit the stories they wish to tell.


Because the original writer does not wish to spend his limited time writing about magical girls and whatnot.

@thewiru
I only know the 3rd one. Would you care to provide a source for the other 2?
The 3rd one is technically both magic and a girl, but she is indistinguishable from Fate/Stay Night mages and nothing at all like Prisma Illya.

The first is from hollow ataraxia.
The second is Phantas-Moon, that page being from character material.

@thewiru
I only know the 3rd one. Would you care to provide a source for the other 2?
The 3rd one is technically both magic and a girl, but she is indistinguishable from Fate/Stay Night mages and nothing at all like Prisma Illya.

@SuperAdventure So from what I can tell, every Fate series is based on the same setting or circumstances, just with a different touch on what they want to tell. It's like different authors reusing the same super-figures or villains to make their own story.

Thanks for trying to simplify it, because when I really think about it, it's like some weird superhero franchise with their own respective origin story coming to terms with the superheroes or supervillains.

@Lucifrost
Yeah, that one in particular was a mistake of mine.
Regardless, it's a bit arbitrary to not count MahoYo, here.

The first image is from Rin using Magical Ruby.
Fate/kaleid liner Prisma☆Illya's premise that is that Magical Ruby abandons Rin and chooses to partner with Illiya, so for all intents and purposes... those are Fate/Stay Night mages, the difference being that the ones in MahoYo are using casual clothing.

@SuperAdventure So from what I can tell, every Fate series is based on the same setting or circumstances, just with a different touch on what they want to tell. It's like different authors reusing the same super-figures or villains to make their own story.

Thanks for trying to simplify it, because when I really think about it, it's like some weird superhero franchise with their own respective origin story coming to terms with the superheroes or supervillains.