

Every custom, every belief, every fashion, every turn of speech?
No, of course not. Why would anyone waste effort on infinite irrelevant details? But everything there is to know, I know.
I do believe that player should be able to gain a basic understanding of the cultures their characters come from. The question is how much information can they get, and process?
You give them an overview at the start with the information you guess might be relevant or interesting to them, and supplement it during the game as necessary.

Science Fantasy is usually a fantasy story in a setting typically associated with scifi. The classic example is Star Wars; it’s it a world with spaceships and lasers, but it’s about space wizards having swordfights.