A source (CD player, DVD, Blu Ray, Computer, etc.) digital output connects to the DAC via USB, Coax, or Optical cable (most DACs support all three digital input types). The DAC outputs to a preamp input via RCA cables (some have balanced outputs, but the Schiit Modi and the SMSL SU-1 have standard RCA outputs, right and left channels).
Also, both of those are powered from a standard 5V phone charger to a USB-C port on the DAC.