Normally on the original version, you can go into settings to find a 'file share' menu where you can look around in the community to find maps. In the MCC version, you go onto your friend's/fireteam member's spartan profile and look in the file share menu.
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go to local files and somewhere in there you can find peoples forge maps based on which maps they are made in such as forge world or sword base
Edit: Reach also has a custom search where you can search for specific map/game names or other player's file shares containing maps, it can also categorise the type of game or map you are looking for. However, to do all this you must have 'Xbox live' with internet connection, if you don't, then you wont be able to go on-line and most of these features will be inaccessible. sorry=(
you go to recent maps if its not their then its gone sorry.
Well you can't save the world on everybody edits cause its just a game where you can build and do other peoples maps:)
Yes, you do.
YES! Halo reach has a "Forge" feature, which allows players to create maps, the reccomended map is forge world.
No. I'm sorry 2 say that I cannot possibly answer this question, as I have just recently started playing halo reach and do not know a lot about it.