I have setup adb server and client within the Android device and can issue adb commands via terminal emulator.
But I need to have the fastboot in the same way. My requirement is to temporarily boot into TWRP without a PC or another Android device:
# fastboot boot /path/to/twrp.img
I think the Android has fastboot-over-usb.
You can't execute fastboot commanda without a physical connection to the phone.
Fastboot is another name for "bootloader mode", where the bootloader stops itself before further loading the OS, and waits for fastboot commands.
Since bootloader is very simple, your only bet would be connecting your phone to a computer.