It should send the video signal to the screen connected to the dock.
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The USB-C/TB3 output might be wired to the Intel GPU or the NVIDIA GPU in that system. If you check NVIDIA Control Panel's multi-display setup area, I believe it shows which outputs are wired to which GPU. However, even if it's wired to the Intel GPU, you'd still be able to get most of the benefit of the NVIDIA GPU through Optimus technology, which basically uses the NVIDIA GPU as a render-only device and has it pass completed video frames to the Intel GPU for output to the display. In fact, that's probably how the NVIDIA GPU works on the laptop's own built-in display, because almost all systems have the built-in display wired to the Intel GPU. The main benefits to having an output wired directly the NVIDIA GPU is that running through the Intel GPU prevents using certain technologies that might be supported by the NVIDIA GPU but not the Intel GPU, such as VR, G-Sync, 5K resolution, stereoscopic 3D, etc. And SOME applications don't seem to work well with Optimus, but many do.
The one thing you want to AVOID is a dock that uses DisplayLink technology (not to be confused with DisplayPort, which is fine). If you're looking at Dell docks, this means avoiding the D3000, D3100, and D6000. They're a problem because they use DisplayLink instead of tapping into native GPU outputs. But the WD15 and TB16 tap into native GPU outputs, so they'd be fine.