How casting work?

Hello, I am using Symfonium to cast music to a Wiim Ultra using either UPnP or Chromecast.

My music library is stored on a NAS (a USB drive connected to my router).

I have a question about how playback works in this setup:

Does Symfonium stream the audio data directly to the WiiM Ultra over Wi-Fi, or does WiiM Ultra fetch the files itself directly from the NAS over the local network?

The reason I’m asking is that playback is sometimes unstable. My Wi-Fi signal in the listening room is quite weak, although the WiiM Ultra itself is connected to the router via Ethernet.

For context, playback works perfectly if I use the WiiM Home app directly, but I much prefer the Symfonium app.

Thanks.