Jitter can be added two ways - either during transmission of digital signal that can influence D/A clock or by inducing electrical noise, that produces time jitter (point in time of signal crossing threshold changes when signal has added noise). In case of Ethernet, USB etc - your DAC gets bit perfect signal and your DAC clock is independent, but cable injects induced electrical noise, that in effect produces jitter. Specification of Ethernet, for instance, calls for isolation of data, but when it is typically done with transformers it will still conduct highest frequency electrical noise thru transformer’s capacitance. I’m sure there are separating devices for that.