Client-side apps are saved to your browser's local storage, and therefore are not saved to the website. Other devices cannot share this data with eachother unless you export and import that data yourself.

Server-side apps are saved to the website's database. For your safety it's advised not to store any crucial personal information like passwords, etc on these apps. Your user account password is encrypted but is also advised to use a unnique one in-case of data breaches.