... I erroneously believed UDP had been adopted by streaming services by now ...
Some may still use it. It wouldn't surprise me if both Spotify and Netflix use UDP protocol - each of them are lossy. But Qobuz and Tidal are TCP/IP and bit-perfect, for sure.
... then what is it in the digital signal that gets "scrubbed" or cleaned up in the Digital to Digital Converter?
Dunno. I don't use a DDC.