On the gaming models (of which this is one), the external port is hardwired to the nVidia GPU.  There is no way to disable the nVidia GPU for the external display.

Unless there's a setting in the software to do the switch (this system doesn't have hardware-controlled hybrid video), there's no way to do what you want (short of buying a system that has hardware-controlled hybrid video;  the Precision and Alienware systems do, but the Inspiron and XPS systems do not).

If the NVIDIA graphics card is enabled while the external display is attached, it's because the display output you're using is directly wired to the NVIDIA graphics card rather than the Intel graphics card, in which case you wouldn't be able to disable it.  On most systems, all display outputs are wired to the Intel graphics card and the NVIDIA card only kicks in when needed, and then it acts as a render-only device that passes completed video frames to the Intel GPU so that it can pass them to the displays.  However, I believe on the Inspiron 7567, the HDMI output is wired directly to the NVIDIA graphics card, because this design allows users to try technologies that the NVIDIA GPU supports but that the Intel GPU does not support passing through, like VR.  One option you could try though would be connecting the display to the system's USB-C output using a USB-C to HDMI cable or dongle, since I'm fairly sure the USB-C output is wired to the Intel GPU instead.

Do you have any idea why my ssd (samsung 960 pro 512Gb) has very high temp (> 52 celsius degree) when I connect my laptop to external monitor? When it is not connected, the temp of ssd is normal (~40).

Thank you all.