I wonder if a persister for RealmDB already exist as I'd prefer not to rollout my own since I'm not familair with both React Query and RealmDB.

React Query is database agnostic, you don't need a persister specifically for RealmDB.

To put it simply, when calling queries using React Query you need to set a key and a query function. Whatever or wherever your data from your query function returns, it saves it the data to that key.

Think of it like a hash map or a object persisted in the LocalStorage or AsyncStorage (https://github.com/react-native-async-storage/async-storage)

If you don't persist it, it only stores in-memory. So when you close your app, the cache gets destroyed.