By providing more backstories and explanations. Let's say there's a magical world in a movie, but the movie only has time to show a little bit of how the magic works. A novelization can go into great lengths about the origin of the magic, the rules governing it, and how different characters interact with it. This not only enriches the existing story but also expands the boundaries of the fictional universe, making it more immersive for the readers.
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.