I know a little about Orks and their weird group psychic thing where painting their ships red to go fast really makes them faster, and their tech only works because they think it should. But then I guess Orks aren’t even in this game.
I get that it’s a violent, black-vs-grey universe. I think it’s the originator of the term grimdark? The emperor is some immortal Mr House asshole who’s worshipped as a god, which powers some of their tech and protects them from chaos via his psyker shit? And he’s kind of a fascist, but it’s that or bloody chaos?
Rogue traders seem like somewhere between a privateer and a baron, plundering tithes in their castle-ships and acting as an arm of the Imperium in backwater space?
There are more wizards than I expected. Not sure if these mechanicus dudes are pulling “Temples of Syrinx” thing, or they know how the tech works but not why so they worship it? Or if it’s actually machine spirits?
Anything super foundational that I’m missing?
Look up the Emperor and his origins which also leads you into what the Immaterium/Warp is as well as the Astrnomicom.
From there you’ll go to the Horus Heresy to understand why things are messed up with some light reading on the various theologies that dominated the Imperium. As well this also leads into understanding what Chaos is, especially to the Imperium.
The Imperium isn’t just fascist, it’s fascist to the extreme with unending bureaucracy. Entire planets get misplaced in the archive as rounding errors. You’ll learn about Space Marines, the Imperial Guard from here.
Then look up the main Xenos factions. I recommend Orks, Eldar, and Tau as those are the current major players/antagonists to the Imperium. There’s more but those usually get more screen time so to speak.