Nope, Superman isn't a Marvel comic. He's a key figure in the DC Comics world. DC has its own unique style and set of characters, and Superman is a big part of that.
Superman could fit into the Marvel Universe in several ways in fanfiction. Firstly, his origin could be tweaked a bit. He could be an experiment gone wrong in a Stark Industries lab that gave him his powers. This would immediately connect him to the Marvel world. His powers would still be his trademark ones like flight, super strength, etc. He could then become a sort of protector of a particular city in the Marvel Universe, like New York. He would interact with the local heroes and villains. His presence would disrupt the power balance a bit, and some villains might target him specifically. But with his unwavering moral compass, he would stand up to them and become an important part of the Marvel Universe's tapestry of heroes.
Superman is definitely a DC Comics character. DC has built a rich and extensive mythology around him over the years. He's a symbol of hope and justice in the DC world.
In a Superman - Spider - Man crossover fanfiction, their meeting would be a clash of two different hero archetypes. Superman is the all - American boy - scout type, while Spider - Man is the quippy, relatable neighborhood hero. They might start off on the wrong foot, with Superman perhaps seeing Spider - Man's more light - hearted approach as irresponsible at first. However, as they face a powerful threat together, like a super - villain with the ability to manipulate time or reality, they would have to put their differences aside. Spider - Man could use his quick thinking and scientific knowledge to come up with a plan, and Superman would use his raw power to execute it. It would be a story of two heroes from different worlds learning to work together.