As this may become a closed circuit, so will it make my motherboard dead? or nothing will happen. How os will act on this internally?
USB Specification states that before any power transfer or data transfer goes on, a handshake has to be approved first.
This basically means that when you connect the cable as suggested, the system itself will do 2 things: ask for a connection and receive a connection. Because it knows what kind of device it is, it is quite likely that it will do nothing at all. In either case, it will not send any current through the device that is strong enough to kill your device. If your laptop can be powered by an USB-C connection, then your laptop will also be protected by receiving that power from itself. If your laptop cannot be charged, it will simply not accept the connection.
It is likely that it will realize the device its requesting from is itself and deny the connection based on that assessment alone.
It will be safe to do. In worst case, it will drain the battery slightly faster than usual.
The answer depends on whether the ports in question incorporate the necessary sophisticated circuitry which prevents electric surges and on the quality of the USB-C cable.
It is well-known that connecting two computers by a simple USB cable (as opposed to a USB link cable meant to link two computers together), could potentially short out the computer’s power supply for good and possibly cause a fire, depending on the ports and cable.
A cable that is meant to connect computers, a link cable, will have a resistance built-in to stop power surges. For example, see the Belkin Easy Transfer Cable:
If you wish to know what can happen to a USB port to which is plugged the wrong cable, a famous example is what happened to Google engineer Benson Leung, who in the course of testing a USB-C cable destroyed his Chromebook Pixel, and it happened instantly.
So my advice is basically not to do that, unless you are willing to sacrifice your device in the spirit of scientific inquiry.