This is an offline single-HTML app. No data goes to or from the internet after this page is loaded. Nothing you enter is ever transmitted to a server. Your spreadsheets live in your web browser's built-in database — on this device, in this browser, and nowhere else.
Your data is YOUR responsibility.
Clearing browser data, switching browsers, using private/incognito mode, or losing this device erases everything. Use Export in the top toolbar — or press Ctrl/⌘ S — to save a JSON backup somewhere safe.