Other lists can be used like a template, or list of ingredients, from which items can be chosen.
Tap the plus (+) button above the keyboard:
This will open the drawer:
Each list is shown in its natural colour (See How to change list colour)
Select the list by tapping it:
Add single items to your main list using the (+) button.
Or, add everything using the Add All button
Changing the order of lists
The order of the lists can be changed using the sort icon: