How to correctly implement the download, storage and updating of user data?

Question

Android

Hello.
Please tell us how in mobile applications (both platforms are of interest) it is correct to implement the storage and updating of user data?
Explanation: there is an application where the user can create / receive applications and send / receive messages (messenger in the application). Most likely, it is wrong to download all requests and messages every time, so you need to store it locally, right? If so, at what point do I need to download this data and how to update it, that is, at what points? The first thing that comes to mind is to get the number of admissible applications when opening the application, check with the number in the local storage, if they do not match, download from the database and write to the local storage. But not too hard and long is it? How do you solve this problem in your projects? Thanks