Hate to give an answer that most of the time seems obvious/snarky, but most of it comes from Flea's fingers. One of the few things I do on this miserable rock reasonably well is play bass. Sounds perfect and seamless when done with skill, amateur hour when not nailed with precision.
Tight Motown-style eighth note finger pumping, not too close to the bridge for just enough body, not too close to the neck so it's not flabby, add compression to flavor. New strings can have too much harmonic overtones, so roll off the treble a bit.
Nothing spectacular there, just classic tone and technique