For context, my brother in law is a huge 40k fan, he has a bunch of minis, paints them all, plays the tabletop game, etc. I only discovered the Warhammer Lorecast on spotify yesterday, which is only on episode 7 currently, but it sounded cool as hell.

So far, I don't know much, except the Emporer had genetic "sons" made, like test tube babies, (at least I think, I'm not even sure if he treated them like actual sons, or just random-ass babies that were raised as military comanders?) called Primarchs, who are the leaders of the different Legions. Who were all kidnapped (?) by Chaos Gods? I think?

I know Horus was the first one he found/recovered, and was his favorite. I know of something called The Horus Heresy. (Don't know much about that, except that later on the Emporer is known as the God Emporer, or the Corpse Emporer, and is very much worshipped as a god) and I think there are something like 12 or 13 "main" space marine legions.

I know the Grey Knights (my BIL's army he plays) sounds cool, so do Space Wolves. And Blood Angels sound cool too.

I guess my question is, where in the hell do I start reading about the lore, where do I learn about the different legions, who's more badass, and why is it (again, at least as far as I know) only the Space Wolves that answer directly to The Emporer and no one else, and can essentially tell everyone else, including the Inquisitiors basically to fuck off?