Where did bethesda get this lore filled world?
Did someone already make it for them or did
they make it from scratch?

They developed it themselves.

what he said

They started making written lore with Daggerfall, which I believe they started developing back in 1994. At that stage, however, Elder Scrolls lore still seemed heavily inspired and influenced by existing fantasy stereotypes, but Bethesda made it all.

The idea of the world orginally came from a game of DnD that the orginal developers used to play. The lore didn't start to flesh out until daggerfall and didn't get extreamly deep until morrowind.

/\ it was somewhat in depth in daggerfall, its just that morrowind reconned a lort if it to make way for intellectual discussion.

Schenectady, New York.

(An old joke by the SF writer Harlan Ellison, who used to tell people he got his ideas by mailing a letter to Schenectady.)

"Bethesda's" lore comes from several writers, who are more or less inspired.

Here's how it works. Take several dozen real world mythologies and religions. Put them in a Large hadron Collider (wear goggles for that part). Pick up the pieces and start doing the modeling.

No. They pilgrimage to the slopes of mt. Parnassus, where the oracles must be choreographed before them. The lore writers decrypt the steps into names, places, dates, etc.

Target five dollar bin.

But no, seriously, they came up with all of it.

Well..sort of. Not "all of it." It's fantasy genre. It's very contrived, overall. But not nearly as cliche' as some series' lore.

In Arena it was pretty basic stuff actually. Could have passed for a decently made DnD campaign, but it was very cliche'. At that time most everything's pretty much what you'd expect it to be. Orcs were just bloodthirsty monsters for instance. It wasn't until Daggerfall and especially Morrowind that things really began to take on their own flavor.

I suspect such expansive and unique worlds take time, the collaboration of talented minds, and in MK's case several mind altering substances in order to acquire all the elements that make them what they are.

Inspiration comes through the course of naturally (if you believe in such a thing) occurring events, which we're told make sense and are called: life.

Functionally the same thing. OR better yet, they do both. At the same time. And don't. Natch.