You guys may have to start with some specific questions... there are so many sub-topics within the general topic.
As far as assignments specifically, there is typically a regular pipeline that a college HC has across a few competitive summer league levels that is his first go-to. From the other end, people from the summer league organizations are reaching out to college coaches for players. Some college HC's are all over this early, some leave it more up to the players, and everywhere in between. Most often, it is expected that the player follow the HC's lead unless the player has an acceptable (to the HC) alternative option available. Some of the more established leagues already have all or most of their rosters filled out. Some are just beginning. There is typically a flurry of additional activity when college rosters are solidified and again, at the end of Spring when players determine they are not going to fulfill their summer obligation, whether it be due to injury, innings limits, homesick, internships, work opportunities, summer school, etc.