Audio to digital conversion

I read with interest macdobr's solution to digitizing audio output from a cassette recorder using Garage Band.

I too bought an RCA to USB-C cable unaware that it was DAC which only works in converting digital to analog applications. I have not been able to find an ADC cable for purchase. Would appreciate anyone who has successfully found an RCA to USB-C ADC for purchase to let us know where/from whom it was purchased.