Mr m, dedicated phono cables meant only to be used between a tonearm and a phono stage input, usually have a female DIN plug at the tonearm end. A DIN plug mates with a male DIN to be found at the base of the tonearm, and it contains five connections, two for right channel hot and ground, two more for left channel hot and ground, and the fifth connection makes contact with the body of the tonearm. That fifth connection is what you see as an external ground wire, and it is meant to be connected to the external ground lug on a typical phono stage. Some tonearm/turntable combinations offer a pair of RCA outputs. In that case one can use conventional ICs. As you noted, those don’t give an external ground connection per se. In that case the only grounds are the Audio grounds. If your tonearm offers RCA outputs, then you might consider adding a secondary wire that you can attach anywhere on the tonearm or to a metal part of the turntable and then to the ground lug on the phono stage. Sometimes that is not needed. In fact, sometimes you’re better off without it.