The game is built on the same feudal social and political structures as the early middle ages europe to start off with.
So armies are raised and maintained exactly as they did back then. Smal standing armies maintained by Barons, Dukes and Kings. Counts, Dukes and Kings also have their own household forces. Wealthy landed lower aristocracy and gentry arm and armour themselves. So you get a small pool of regular troops, a small pool of well equipped (but often undisciplined) knights and gentry militia. Larger towns and cities also often had their own militia that acted both as a police force, and a defence force, that at a push could be used in regular military action.
The vast bulk though are levies. So when levies are called the counts go around their towns and villages and gather up a certain percentage of able bodied men, arm and armour them. If a Duke calls their levies then each count provides an agreed upon amount of levies from their county domain. A king calls the dukes who provide an agreed amount of their levies etc. and these pool together gradually to form an army.
So the bulk of the forces are often your peasants, armed with basic gambeson or whatever armour was available and spears or bows. Your regulars, gentry and middle class levies form the better quality troops, aristocracy form your knights, other cavalry etc. or heavy foot troops.
When the conflict is over, everyone makes their way home and goes back to the job they had previously.
Maintenance for armies is generally dependent on who raised the levies or whos personal guard they are etc.
For the peasants, it is their local count. For town militia the mayor. For barony regulars, the baron (with the help of the duke). For gentry and all aristocracy they maintain themselves. Household guards are maintained by the lord's household budget.
For big campaigns the lords would work together to ensure the armies remain fed, often whoever called the conflict at the top level would take charge of payments for food and basic supplies, while each "camp" within the army maintain their own tents, horses and fighting equipment.