Why cannot discover user in Airdrop when open personal hotspot? When my iPhone opens the personal hotspot, I cannot find other users via Airdrop. What is the theory behind it?
AirDrop is different than a personal hotspot.  

A personal hotspot is for sharing your internet connection.

AirDrop has separate controls, located in the control center.   You must also turn on Bluetooth and wifi.

Official Apple doc with screenshots.