We are independent contractors, unless you're working professional baseball under the MLB flag then you're employed by the league even if you're only working minor league baseball. So, I am a member of an umpire association in my area that assigns me games. The association is contracted by travel tournament directors, HS/college athletic directors, ect to get them umpires for a tournament/games. They then assign to members of the association based on experience and more importantly your skill level.

It's also about who you know for sure. I wouldn't be where I am today or assigned the games I get if I didn't know someone who knew someone who knew someone. It's a lot of networking and politics too, which is why we have some older umpires in our association still getting high level assignments that they shouldn't be getting.