I would like to better understand how medieval leaders raised large amounts of infantry. I have done some more or less surface-level research, but haven't found a good enough answer.

High medieval armies were famously cavalry-heavy, but usually still included large numbers of foot troops containing melee and ranged fighters. Towards the late Middle Ages their importance grew once again but moved towards an army of professional payed troops.

I'd like to know how a feudal system produced foot troops in say England in 12th to 15th centuries. It's easy to follow where cavalry came as everyone has been taught how nobles earned their fiefs by equipping horsemen or giving someone a smaller fief to be able to afford it on their own (knights). However the closest to understanding the logistics on the infantry side is that "every able bodied man between ages 16 and 60 could be levied to battle with weapons and armor according to their wealth".

Most of us propably are familiar with the pop-culture idea of a medieval levy, but this is often vague. To my understanding this was primarily a defensive system for locals to organise in home defense. Still I'm confused about who even would organize such a levy and who selects who goes and who does because clearly every able bodied man between 16 and 60 has never been called to battle. Did knights have their vassals, the peasants, as a recruiting pool? Did they make agreements for foot men to be provided to war? Please someone tell me.