Man, Steampunk just does nothing for me unless its like The Order 1886 and its alt reality stuff still in that time period. Really could not care about those skins, New Armor is cool I guess, I'd like a technical explanation on how its possible (fyrewulff?) The Pit is a solid map but I'd have preferred guardian and the other new maps looks okay I guess.
You just(tm) add the new assets to the .map file, then create UI in the patch that lets you select the assets you added.
They didn't do it before because
a) doing so makes the map's file signature no longer match, which means
ab) The game will no longer see them as the same map
ac) which means it breaks LAN, which is something Bungie apparently avoided.
ad) the other boxes can't see the new assets even if you did trick it into loading the map anyway, and will explode / set your house on fire / melt
also b) you have memory concerns.
The advantage of not having LAN compat (Halo 4, Reach TU) is you can add assets and just tell everyone to patch to latest if they want to continue to play together. The downside is you lose the ease of setting up a LAN (in h2 or h3, all you needed was the host box patched - all the clients would act like the later version once connected to it). The upside is you have a lot more freedom to fix and change things that would normally cause desync, like client-side stuff and certain animation issues. And 343 is finally using this.
Bungie did this already actually. They added a new weapon in Sandbox, but it was stuck only playable on Sandbox. They added new vehicles with the Transport Hornet, the Soccerball and the Golfball, but they're also stuck on the maps they were added onto. In Reach, Bungie added the special Elite Purple Reign Bomb for Breakpoint, but it was stuck on Breakpoint only. They could have
in theory retroactively added those to the shipping Halo 3 or Reach maps, due to how the engine is set up.
Then 343 added the changeable Magnum, DMR, and NR, which had certain client-side behaviors (zero reticule bloom, and more notable the Magnum's RoF change) that required upping the game version number and breaking LAN. The Anniversary magnum actually works on every single Reach multiplayer map, because it hotpatches what essentially is a new magnum into all of them.
They apparently tried to add the new Reach armor, but it fell through for reasons we don't know.
The downside to adding armor, especially in Reach, is you have to patch it into every single .map that players can customize their armor on. In Reach, this was EVERYWHERE (campaign, multiplayer, firefight), which means you have to find memory in every single inch of the game to stick that helmet into. And some of the campaign maps alone, according to Bungie employees in unrelated interviews, were redlining already. It's a possible reason why the new Reach armor died on the vine. Either they didn't have enough memory or not enough patch file size (Reach seems to be subject to the old 4MB patch limit) or it wasn't worth the risk:reward.
Halo 4 has the advantage that customization is not available in Campaign, so they can ignore that part of the game completely. They only need to patch in the new assets to Spartan Ops and Multiplayer Maps, and Halo 4 is compiled with the newer SDK that stores patches permanently and can have anywhere from 4MB to 3GB patches. They have a lot more breathing room in multiplayer and Spops maps. Plus, they don't have to worry about film compatibility for SPOPS, only mutliplayer, which further reduces their test load to do this.
tl;dr the TU adds the new armor and weapons (I believe Halo 4 treats skinned weapons as their own weap and not just a new tex on the same gun model). To use them, you're going to be buying a software key to let you select them in the UI so everyone else playing Halo 4 can see you wearing the armor. The maps themselves are DLC data you're going to have to download.