Google Drive File Stream just doesn't work out of the box on Big Sur due to the way that kexts are loaded.
Step One: Launch Terminal
Step Two: Run in terminal
curl https://raw.githubusercontent.com/mtan93/googledrivefilestream-bigsur/master/fix-bs-filestream.sh -o ~/fix-bs-filestream.sh && chmod +x ~/fix-bs-filestream.sh
Step Three: Run the script in terminal by pasting ~/fix-bs-filestream.sh
Step Four: Then allow the dfsfuse kext under System Preferences > Security & Privacy and reboot your Mac.
Step Five: Run the script again by pasting ~/fix-bs-filestream.sh
Finally the mount should pop up on the desktop!
Credits: slw07g - the initial work-around which needed Google Drive File Stream to be killed before the commands executed.