Most coaches run camps on their campuses. If they're not running one, they're working someone else's. It is highly recommended that you attend camps where college coaches will get to see you play. Not only will coaches get to meet you and watch you play, but you get to meet the coach and experience that coach's philosophy and techniques. Coaches can not possibly see every athlete compete and can't possibly know about every great athlete out there.
Some college coaches get recommendations from club and high school coaches.
Others receive tips from alumni who will let coaches know of talented athletes in their hometown area. Even college coaches share information with each other about recruits. I and II schools can offer athletic scholarships, and these coaches are likely to have recruiting budgets. Also, these coaches are more likely to be active in pursuing athletes in the recruiting process. Division III, NAIA and junior colleges do not have athletic scholarships and, typically, do not have large recruiting budgets.