Was it hereditary? Was there a formal application process like today? Would they just be selected by nobles with no way of refusing?
For that matter, was it the kind of job that might be sought out by a lower-class person or was it no better than any other job available to them?
I'm curious about lots of things related to historical job searching but I don't want my question to be over-broad. Nonetheless, if your area of expertise is about a different time period, region, or type of job, I'm still interested in the answer.
The term "medieval" covers a thousand years, so I wouldn't dare to pronounce on the whole of it. But there is an answer for the pre-industrial world that would likely include a lot of the medieval period: they often started out being brought to an employer by their parents.
GOBBO (to BASSANIO) God bless your worship!
BASSANIO Wouldst thou aught with me?
GOBBO Here’s my son, sir, a poor boy—
LAUNCELOT Not a poor boy, sir, but the rich Jew’s man that would, sir, as my father shall specify—
GOBBO He hath a great infection, sir, as one would say, to serve—
On reaching adolescence, a child became big enough to do some serious work, and also often became less obedient to his or her parents. Even back then, it was noticed that teenagers tended to behave better when over at someone else's house. It was therefore very common to put them into another household to learn to serve. That could mean something pretty equal, like a tenant farmer putting his daughter with another tenant farmer- and that other tenant farmer would also perhaps send over his daughter at some point . Or the farmer could go up to the local lord's house and see if the butler or majordomo had a place for her as a servant. The length of time they served would vary: after some years of accepting vails ( gifts from visitors) a girl might have enough money saved to make up a dowry, which would make her marriageable , and she could leave. But she also might have skills: become an excellent seamstress, or gotten clever with cooking. She might decide to just continue as a servant. A boy might do the same, show himself to be good with horses or hawks, or just be quick with bringing in buckets of water from the well. He might become a part of the staff.
This would also be the time in which a boy would be apprenticed, and that was a similar process: he would move out to the family house and into another household, where he would learn a trade. And, as with the regular servants, he might stay on as a journeyman after serving his apprenticeship.
Peter Laslett : The World We Have Lost