The DAC has to be physically connected - usually indirectly via the amplification stage - to the speakers, so if you have speakers or headphones attached to your PC then the DAC in the PC will be used.
However, if you're using the WiiM/VLC app on your phone to play music to your WiiM Pro Plus (which has a DAC), then it will be the Pro Plus that performs the conversion, and everything before that will be in the digital domain.
You could even just connect your PC (analog out) to an amplifier and use the internal DAC, although it likely won't sound as good as an external DAC.