XPS 15 9550, Nvidia GPU, run external monitors

On the XPS 15 9550, all outputs are wired to the Intel GPU.  And on all Dell systems I've seen as of this writing, the USB-C output is wired to the Intel GPU even if other outputs are wired to the NVIDIA GPU -- with the exception of newer Precision 7000 Series models that have a BIOS option that allows you to choose which GPU controls the display outputs.

I've read conflicting reports about the 9570.  I've seen some posts indicating that the USB-C output is still wired to the Intel GPU but the HDMI output is wired directly to the dGPU, and others indicating the opposite.  The way to find out is to open NVIDIA Control Panel and go to either the multi-display setup area or the PhysX setup area.  One of those should have a diagram showing all display outputs in the system and indicating which GPU each one is wired to.  You might have to actually have a display attached to the output for it to appear, but that will tell you.

All that said, in most cases you don't need your display hardwired to the dGPU.  The exceptions would be for things like running VR, 5K resolution, and stereoscopic 3D -- but the 95570 wouldn't be any good for VR and the other 2 are pretty uncommon.  A recent thread here also suggests that Adaptive V-Sync might not be available when running through the Intel GPU, which I guess could be annoying.

@buggaby  attaching a display to the USB-C/TB3 port is completely different from attaching an eGPU.  When you're using that port for one or more displays, the USB-C/TB3 port is carrying a DisplayPort video signal that was rendered by the Intel GPU.  When you're using a Thunderbolt eGPU, that port is carrying a PCI Express signal that doesn't carry rendered video at all.  Instead, that type of connection is functionally equivalent to installing a PCI Express card internally on a desktop.

But in terms of whether the Intel GPU can still be involved with an eGPU, it depends.  If you connect the display you're using directly to the eGPU in its enclosure, then the Intel GPU would not be involved, because in that case you have a way to attach your display directly to the eGPU.  If on the other you use the eGPU to accelerate content being displayed on the laptop's built-in display, which is wired to the Intel GPU, then yes the Intel GPU would be involved because it has to be.  In the latter case, the eGPU would work just like the dGPU would -- as a render-only device that has to pass completed video frames to the Intel GPU that is physically wired to the display you're using.  The fact that the eGPU is external and the dGPU is internal doesn't make a difference to how they would operate.  Again, the fundamental question in all of these cases is, "Which GPU is physically wired to the display output that my display is connected to?"