Armies had a lot of more or less professional infantrymen, be they in the retinue of a lord or knight (and they could be peasants even if a peasant would generally have no reason to go to war, and the lords would have few motivations/legal means to constrain serfs to go), mercenaries (https://en.wikipedia.org/wiki/Free_company), or common-law prisoners (a bit as the Russians did in Ukraine).
Free families would often have to send someone to war if the local lord called.
Free families would often have to send someone to war if the local lord called.