How to properly organize the structure in MobX?

Question

React

Good evening!

Tell me how to properly organize the data storage structure in Mobx.
For example, demo ToDo.
What is on the Rest server:
1) tasks
2) projects
3) Each task is associated with a project (project_id).

If we consider a situation in which a browser application and a mobile application (React Native + MobX) knock on the same server - that is, data synchronization and the data update algorithm are important (when to download and synchronize on the client, how to synchronize if the client stores the data of one user, and server data of all users when send for synchronization on the server).

And actually interested in how to properly store table relationships in MobX?
I understand well the principle of data storage and their use on the Postgres backend.
But as I understand it, the situation is different on the client, because there you need to store data for offline access + somehow synchronize them.

Tell me how to do everything right?