By default your app keeps its data on the device you're using. That suits most homemade apps: it's fast, it works with no signal, and nothing you log leaves your phone.
It also means the data stays there. Open the same app on your laptop and you'll start with an empty list, and clearing your browser data can wipe it. For a packing list that lives on your phone, that's fine. For anything you'd be upset to lose, it isn't.
When you want more than that, just ask. Tell the AI you want your data saved online so it works across your devices, or shared with other people, and it will help you set up a real database and walk you through connecting it. You'll be asked for a couple of keys along the way, which we store securely for you.
It's worth deciding this early, because it changes how the app is built. Two questions to answer when you describe it: do you need the same data on more than one device, and if other people use it, does everyone share one list or does each person keep their own?