The Migrant Fleet is the largest concentration of starfaring vessels in the galaxy, sprawling across millions of kilometers. It can take days for the entire Fleet to pass through a mass relay.
When the quarians fled their homeworld, the Fleet was a motley aggregation of freighters, shuttles, industrial vessels, and the odd warship. After three centuries, all have been modified to support larger crews as comfortably as possible. As the quarians achieved stability, they began weeding out the ships least suitable for long-term habitation, selling them and pooling the money to buy larger and more spaceworthy hulls. This process is ongoing, as vessels wear out and break down.
While some ships enjoy dedicated cabins with full privacy and sanitary facilities, many more are former freighters, whose cargo bays and containers are pressurized and divided into family spaces using simple metal 'cubicle' bulkheads. The quarians enliven these austere spaces with colorful quilts and tapestries, which also help muffle sound.
The day-to-day operations of the fleet - traffic control, station-keeping, supply distribution, and so on - are under military jurisdiction. Though ship captains have the authority to deviate from their assigned positions and may leave the fleet at any time, they are assumed to do so at their own risk. As the Migrant Fleet moves around the galaxy, many ships split off to pursue individual goals, returning days or years later.