Tapping and holding the Home button will start the Google Assistant app, and in addition to "OK Google", the Assistant app on the Home button can also be turned off.

Android 9 | LG UX 9.0
Step 1:  Open the  Settings
Step 2:  Tap on  General
Step 3:  Tap on  Apps & notifications
Step 4:  Tap on  Default apps
Step 5:  Tap on  Assist & voice input
Step 6:  Tap on  Assist app
Step 7:  Tap on  None
  1. Open the Settings
  2. Tap on General
  3. Tap on Apps & notifications
  4. Tap on Default apps
  5. Tap on Assist & voice input
  6. Tap on Assist app
  7. Tap on None

LG Instructions