How does offline-first database design work?

I would like to make an offline-first To-do app which works on both the mobile app (SQLite) and the web app (PostgreSQL). The app will have these business models: User: - id - name - created_datetime Todo: - id - user_id - title - done - created_datetime - updated_datetime Creating new users: - Only can create a new user with the web app. Creating new to-dos: - Can create to-do with the web app. - If the mobile app is online, post the to-do on the web app. If the post was successful, create the to-do with the same successful to-do's id in the mobile app as well. - If the mobile app is offline, still create the new to-do, and update/post the new to-do to the web app, whenever it is online again. I am unsure how to create a new to-do if the mobile is offline and if I don't have any id, to begin with. If I create a dummy ID for the mobile app, it may clash with other to-do ids. Or should I use UUID for all my to-do's id in the mobile app, and then post it to the web app? How does other offline-first app database design work?