If youre referring to the movie, that is a different thing from the comic, the comic has them literally just having always lived on remnant (Clark and Arthur were schnee dust company workers, victor was a scientist in Atlas, bruce was an atlas rich kid, and barry and diana just existed), whereas the movie has it be the actual justice league, just thrown into the RWBY universe and altered by its (something which is actually weirdly consistent with the current comic run, that RWBY as a universe has law hax that overrides the metaphysical laws of other universes)