Endgame originated from the Marvel Comics universe. So, in a sense, it can be considered related to comic books. It brings those comic book elements to the big screen in an exciting way.
The number of Marvel comic heroes is constantly changing and growing. As of now, there are well over 8000 characters, but new ones are always being introduced.
Not entirely. While it borrows key elements and characters from the comics, there are significant plot differences and creative liberties taken to adapt it for the big screen.