in addition to the power key, the LG smartphone's camera can also be started using the volume down key. the key mapping is disabled by default on the device. once enabled, pressing the volume down key twice can start the camera. this works both with the display on and off.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  General
Step 3:  Tap on  Shortcuts
Step 4:  Enable or Disable  Open Camera
  1. Open the Settings
  2. Tap on General
  3. Tap on Shortcuts
  4. Enable or Disable Open Camera

LG Instructions