SPDIF is spec’ed for coax terminated with 75 ohm RCA.
Most cables marketed as SPDIF are not exactly 75 ohm. In fact, it’s difficult or near impossible to make one to spec unless it’s terminated in BNC.
Video RCA composite (yellow) and video RCA component (three colors) are also spec’ed the same as SPDIF. With yellow, red, white, they all tend to be close enough, too, except for those with a thicker yellow that probably is closest to 75 ohm.
Most cheap audio RCAs are coax and not twisted pair audiophile stuff.
Unbalanced RCA line level is highly tolerant of interconnect impedance until you get to massive amounts (you can get that from long runs), when it starts to attenuate the high frequencies, among other things.
SPDIF is surprisingly tolerant of impedance on shorter runs. Digital audio has actually been demonstrated along wet string!
So you do the math on the likelihood your cheap line-level audio RCA cables deviate that much from SPDIF specs to matter. Economies of scale for making RCA cables of all types means that most of them are close enough for going player to mixer.
Now, phono cables and capacitance… that’s another can of worms. Beware of people trying to rewire your Technics, for instance.