Got a million questions, this is the first.
Dedicated current and signal paths are created using copper cables of various configurations, depending on use case (simple stranded, twisted-shielded pair, coax, etc.)
The ISS structure, made generally of 2219 aluminum (and a few other alloys here and there) is held at a common ground potential as well via dedicated electrical bonds from piece to piece. This is not an intended return path but instead exists to provide a return path for electrical faults and to ensure that interaction with the LEO plasma environment does not create dangerous potentials between hardware elements that could injure EVA crewmembers or damage sensitive hardware.
To address a comment to the question: silver is not used on ISS, as it is not compatible with the atomic oxygen environment in LEO. Exposed silver will be continually oxidized, and (unlike metals like aluminum) the silver oxide will spall away, eventually eroding away all of the silver.