I follow the big names of 40k lore on YouTube too (Tactica Imperialis, 40k Theories, Arch Warhammer, Luetin09, Grimdark Narrator & Oculus Imperia), but understand, these channels &their content are there to get you started into 40k.
The fluff/lore is vast and it is continuely being updated and/or retconned; and, often times there is no or differing information.
This is deliberate. Games Workshop sees these differences as stemming from the thousands of years that have passed and the differing biases by the different factions. So everything is canon, but not everything is 100% true, or the complete story.
What I am getting at is make up your own mind, and make the story your own. Don't just parrot what the large names say, okay? This is the main reason I shitpost! I want to get you thinking about it!