By introducing themselves, being clear about their modalities and who they work well with, and being reliable when someone is sent. These relationships are straightforward to build and durable, and reciprocal referrals sustain them.
These are among the easiest referral relationships to build in any profession, because the client need overlaps naturally and neither practitioner competes with the other.
A chiropractor sees people actively trying to address a physical problem. Soft tissue work frequently complements what they do, and many chiropractors would prefer to have a therapist they trust rather than sending patients to search.
What builds it:
The same approach works with physical therapists, personal trainers, doulas and estheticians. Three or four working relationships can supply a meaningful share of a practice's new clients, and they cost nothing to maintain beyond responsiveness.