What type of homeowner hires their plumber/HVAC guy based on "relationships"? They might only need a visit only once in a few years, so the idea of "relationships" is laughable.
If neither of these they fall back to advertising, door-to-door sales, classifieds, google reviews.
The first AC guy at my last house and us had kids in the same third grade classroom. Later when his business changed to commercial work, Charlie put us on to a semi-retired AC guy who kept us running for several years until we moved.
By the way, Charlie was recommended by our neighbor two doors down when we moved in.
Hire a couple of disasters, and you will appreciate that a good relationship with a good contractor/tradesperson is worth an incredible amount of money (and time).