I’ve put this in books as this does pertain to literature although you can get Sci-fi and fantasy movies too.
I’m going to start by saying the difference between normal fiction and fantasy.
Not all fiction is fantasy, fiction just means not real. Sherlock Holmes isn’t a fantasy but it is a fiction.
So what does this mean for Sci-fi?
Well Sci-fi is semi realistic in a sense, the hunger games is dystopian which falls into Sci-fi. The things in that story could happen in a dystopian future or a different timeline. But it’s not real, it’s not fantasy as there is no “magic” or anything remotely fantastical. Nobody has any powers and they all fight with their hands or weapons.
You may have seen Deluge explain why Star Wars isn’t Sci-fi and he explained it pretty well and will help me make my next point (Star Wars isn’t a book this is just to help my point).
There is not much science in star wars at all. Merely taking place in space doesn’t make it a Sci-fi. Taking place in Space may aswell have been like taking place in a magical realm.
If it can be explained by science but isn’t real it’s Sci-fi
If it’s explained my magic or an unknown spiritual connection of sorts it’s fantasy
Feel free to add onto this.