It all starts when Peter Parker gets bitten by a radioactive spider. This gives him amazing powers like super strength, the ability to climb walls, and a 'spider sense'.
It expands the universe by introducing new characters. Fanfiction writers can create their own original characters that interact with Spider - Man. These characters could be other superheroes, villains, or just normal people who get caught up in Spider - Man's world. For example, a new hero who teams up with Spider - Man to fight a common enemy.
Spider - Man would probably be defensive at first. He's used to working alone most of the time, and being interrogated by the Avengers would seem unfair to him. He might crack some jokes to ease the tension, but also firmly state that he's a hero too.
The age of Spider-Man in those comics isn't fixed. It changes based on the plot. As for being dead, typically he isn't, but there might be alternate universes or special arcs where his fate differs.