I am building an application that needs to save data locally, and I hope there could be a button that allows user manually backup their data to iCloud,
I have checked: AsyncStorage, SecureStore, and FileSystem in the documents.
But I found nothing about how to achieve it, any idea?
I suspect you’d have to eject to do this. There are a couple of feature requests that seem related. You should upvote them if the functionality is something you want built in to Expo:
Also, the Expo team is making it less painful to eject these days, so you might want to try ejecting to the Bare workflow in the mean time.