The name at the top of the Lists tab is your active list. Tap it to switch to another list, or choose Manage Lists at the bottom to make a new one. Every list button in the app adds to the active list.
With more than one list, a message at the top says Added to that list after you add something. Tap Change to move it to another list. That list becomes your active list.
To add to a different list without switching, touch and hold an item’s list button and choose Add to that list. On Kitchen cards, it’s in the card’s touch-and-hold menu. For a recipe’s ingredients, pick the list at the top of the screen that shows them; that list becomes your active list.