The Silent Cartographer is Halo’s best single player level: the pacing, the scale — at the time — was breath-taking. Now industrious modders have taken that original level, and rebuilt the entire thing from scratch from the ground up.
You’ve never used these guns and or played these levels in any Halo game, and odds are, you never will.
Halo: Combat Evolved released in fall 2003 for PC. Bungie has moved on — but the community keeps the game alive through mods. Take SPV3, for example.