Might be kind of a silly question, but how did people typically end up becoming mercenaries in the first place? Like is it something freemen chose to go out and do in search of glory/wealth, or is it like most things in the middle ages and you do it because your dad did it, or what? For that matter, knights seem to be the rarest and most expensive kinds of mercenaries for obvious reasons, but what was the origin of most sergeants? How did non-knights get training in mounted combat? Really, how did the non-noble mercs learn to fight at all? Trial and error, prior levy experience, or did mercenary bands provide training to new recruits, etc? And as a small final side note, is the distribution listed in the Mercenary Captain Virtue, of one guy leading 5-20 guys, the most common type of mercenary organization, or were there a lot of bigger groups or solo mercenaries? Curious to learn what I can about this topic.
Around 1220 mercenaries were considered an anomaly - but there were many thousands. Medieval literature condemns them in general, but lords and kings employed them.
Take a look at the Brabançons.
Primarilly they would be raised by a lord as an army and when the fighting stopped and they were released a group of them would decide they preferred being professional fighting men to returning home to be peasants. The exception was something like sergeants which could be either nobles who were dispossessed after being trained or who needed money and had seasons they were not pledged to their lord in which they sold their services as warriors.
Sergeants might also be squires who never got enough cash to pay to be knighted.
A quick google link gives lots of books to look up:
So slightly longer answer: men became mercenaries mostly out of necessity. Broadly, men from poor regions (Wales, Brittany) would take up arms by lack of opportunity at home and would use their archery skills or be skirmishers. Richer, fertile regions (like Brabant)would lack areas to turn into farmlands, so younger sons would take up arms as a mean of making a living and would work as "general" infantry often with poleweapons or spears.
The younger sons of poor nobles whose fathers could not give them a real inheritance would still learn weapon skills as youths and would serve as cavalry if they could afford a horse.
Then you have the desperate, people who ended up in the path of an army, whose lively hood was foraged and who d have no other source of food but joining one of the armies, as camp followers if they had a relevant trade or as knifemen if they didn't.
But short version: younger sons whose father had no inheritance for.
foraged here being a euphemism for "stolen and/or destroyed"
I think that really better describes renaissance mercenaries than medieval mercenaries. During the middle ages armies generally needed large populations to support them, and serfs were considered property of their lords in most parts of Europe. for a peasant to simply decide to become a mercenary would be on par with them deciding to become a lord or knight- a fantasy at best. Training and equipment would be the hard part of someone becoming a mercenary and it wasn't like mercenary companies were in great demand and came around recruiting.
While they were disbanded in 1214, the Brabancons were the archetypal ones. Though we could assume that the Hennuyer, their neighbours were run similarly.
Serfs ran away to cities, where if they could hide out for a year could start claiming citizenship (IIRC there is a section about this in either Lords of Men or c&G). Running away from one's lord was risky if caught, but appealing in the long run, which is why the cities grew despite the terrible death rates there.
This depended on periods, very boom and bust. But in France and England, there was a big demand for them during wars, followed by lulls where they'd just turn to banditry. Both Philippe Auguste and John Lackland used them extensively.
Italy had different though important mercenary traditions, though I am less familiar with them...