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
Step 1:  Open desired  Folder
Step 2:  Tap on  Manage
Step 3:  Select all desired  apps
Step 4:  Confirm with  Done
  1. Open desired Folder
  2. Tap on Manage
  3. Select all desired apps
  4. Confirm with Done

LG Instructions