Tethering Explained

I guess the main point of Tethering is to provide Internet to other devices, so I assume that the cellular antenna gets connectivity from the paid plan and channels the received data to the WLAN antenna that acts as a hotspot. I guess one cannot use Tethering to build mesh networks or the "mesh" is always just the one hop from cellular to WLAN. Also, can you use Tethering to create a hotspot without using cellular nor Internet? This way, the phone could talk to computers in an Ethernet network or act as a server without the need to use Bluetooth. Can I "tether" a computer network connected via USB cable between phone and notebook/Desktop-PC to other mobile devices, effectively using the phone WLAN antenna as an hotspot for the computers? Why can't computer WLAN antennas/NICs act as hotspots or why tend they not to? With a little bit of extra work by the hardware manufacturers, we probably would get more options, but no...