343 doesn't have an online method for downloading files for MCC at the moment, you have to do it in game. When you're in game, you have to open up the roster and select your name, from there you can select "find player", and type in the gamertag of the person who made the map you want and look at their file share.
Some more recent tracks have been made on PC and these currently aren't able to be shared by normal means. We'll be getting some sort of PC file share at some point in the early part of 2021 but for now there's a workaround to share them that's only available on PC. If you're on console the only way to get them would be to have someone else put on the map for you, however this isn't an issue with most tracks you'll find.