If we skip MGA here (it has a special map and setting for its entire campaign) and go for MYZ.
You pick the map to play. Either by choosing one of the map that comes with the book, one homemade (there are some good maps done by players, especially in the g+ group for MYZ). The map is divided into sectors, all unexplored, except for one, your home (the Ark). The goal of the campaign is to explore the map (and, yeah, to find clues for Eden). There is a system in the book that you use to randomize what a sector contains. What kind of terrain, significant ruins, threats and useful (more or less) items. This randomly generated sectors doesn't have any deep background or so. it is meant to be a system to fill out the map. Although the threats (i.e. humanoids, monsters and phenomenons) usually give some suggestion how to play them.
Every now and then you as a GM can place a so called special sector on the map. There are five of those in the rulebook and more in the Zone Compendiums. Technically the rulebook has a sixth special sector, Eden. These special sectors have a backstory, a plot, npcs, items, etc. Like adventure modules for other games. Some of these special sectors have a plot that can be a mini-campaign. They can be used as is (there is a list of suggested events for the special sector) or be tweaked by the GM if needed.
What I do is that I prepare a couple of sectors before the session (takes half an hour to an hour or so). Homemade special sectors if you will. Often not that advanced. Usually a map of a ruined house (found on internet) and a threat and "treasure". After a while I have a lot of home made special sectors to throw in or place whenever I like it (since quite often I make more special sectors than the players explore.
Or to be short. The game is designed to be a no-prep game, but for those of us who likes to prep, can do so. One of the few RPG that you actually can play on the fly.
“Thanks for noticin' me.” - Eeyore