halo 2 doesnt have that advanced map editor,
you can make custom maps, and tis not that hard, map editor has been simplified.
and it is plosible to make custom weapons with h2 unlocked map editor that you can just download, already there is map makers that make custom weapon, like halo 1 assult rifle for halo 2.
there isnt co-op like on xbox version, but there is way to mod your mainmenu.map to be able to loead single player maps in online game, anyone can do that( even someone like me that doesnt understand anything about modding in halo)
and its fun to play singleplayer maps with your friends,
it is also plosible to mod single player maps. like making flyable pelican in metropolis level