MongoDB Atlas Device SDK explorations (formerly the Realm mobile database)
As per their docs
As of September 2024, Device Sync is deprecated for Atlas App Services. Device Sync will reach end-of-life and be removed on September 30, 2025
The first demo app is SyncOddly, which shows an architecture for sharing appointments and notes with other users.
It also lets you choose between using FlexibleSync with a MongoDB Atlas server, or just a local database.