After creating a folder, apps can be added in the same way. As an alternative to tapping and holding, apps can also be added to the folder using the edit function. This is especially useful if several apps are to be moved to a folder at once.
Android 9 | LG UX 9.0
Add apps -  Open desired  Folder
Add apps -  Tap on  Manage
Add apps -  Select all desired  apps
Add apps -  Confirm with  Done
  1. Open desired Folder
  2. Tap on Manage
  3. Select all desired apps
  4. Confirm with Done

LG Instructions