Muzan was ultimately defeated by the collective efforts of Tanjiro, Nezuko, and the other demon slayers. Their teamwork and perseverance were key to ending Muzan's reign of terror in the manga.
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.
Well, muzan x reader fanfiction often explores a very interesting relationship. In such fanfictions, the writer usually dives deep into the dynamic between muzan, who is a complex and often dark character, and the reader - insert, which can be a very exciting exploration of power, mystery, and sometimes a touch of romance. It allows fans to put themselves in a fictional world with muzan and experience different scenarios.
No. Ryomen Sukuna is a fictional character from Japanese mythology and modern manga/anime. He doesn't have a basis in a true historical event or person.
Nope. Ryomen Sukuna has no basis in reality. It's a product of the creative minds of the writers and illustrators who designed a captivating fictional character for the enjoyment of the audience.