Often, there is a theme of an unlikely alliance or relationship. Given that they are from different fictional universes, the fanfic might focus on how they come to meet and form a bond. This could involve a lot of world - building to bridge the gap between their respective stories.
It starts at a specific point in the manga, but I can't recall exactly without looking it up. You might want to check some manga forums or dedicated fan sites for the precise chapter.
Well, some fanfic communities on Reddit might also have 'gojo x makima fanfic'. You can look for subreddits dedicated to the respective anime or manga that these characters are from. People sometimes share their creative works there. However, the availability might vary.
Sukuna does die in the manga. The way it happens is quite dramatic and ties in with many other elements of the story, adding depth and tension to the overall narrative.
Makima's fate in the manga is quite complex and involves a series of dramatic events. She faces powerful opponents and has her own agenda that leads to significant developments.