While holding Volume Down, plug the USB-C cable into your phone (the other end must already be connected to a PC or a power source).
If you need to modify system files, look into EDL Mode (Emergency Download Mode) using QFIL or Octopus Box, but that is an advanced repair process, not standard Fastboot. How to Open Fastboot Mode on LG V50 ThinQ
Check our guide on LG Fastboot Compatibility by Model . While holding Volume Down, plug the USB-C cable