They’re completely different implementations of systems that steam video/audio/inputs.

Valve’s is pretty buggy but has deep integration with Steam and allow NAT traversal, while Sunshine/Moonlight are way more reliable, have features that reduce latency but are pretty barebones as far as features: they just do streaming with no tight integration with what’s being streamed.

And Sunshine is a reverse engineered version of Nvidia’s game stream server, since Nvidia sunset Gamestream a few months ago.