Behind the scenes, the referral process is not very formal. A referral can be in the form of a consultant telling a recruiter that they know someone who could be a good fit. If the recruiter agrees, they will reach out.
Often the recruiter will forward to a different recruiter if they feel the fit might be better elsewhere.
There are also often more formal recruiting programs, where a consultant can use certain internal links and often participate in referral programs (sometimes earning bonuses), but this is just to encourage referrals, not required at all.
Hope this helps. Happy to provide more details if you like.