The best way I’ve discovered 40k lore beyond hearing from a friend and some meme is Luetin09, which has a great narrator voice, consider his channel a huge audio library of alleged facts and stories.

How to make a space Marines: youtu.be/whtHtyAy0Wo?si=FdpfykLdVV4aK8-4

Also, I heavily recommend Arbitor Ian as he has both a nice presentation style, and also talks about the meta aspects (like how things evolved in the 40 years of 40K lore changing)

The timeline of 40k in 20mn: youtu.be/05YRMHWtv1Y?si=xnoLgKkKAQgRwEsX