How do I add security to React Native app?

SSL Pinning in React Native Apps are dependent on Certificate Authorities (CA) and Domain Name Servers (DNS) to validate domains for TLS. Unsafe certificates can be installed on a user device, thereby opening the device to a Man-in-the-Middle attack. SSL pinning can be used to mitigate this risk.12 Ara 2019

A hybrid approach to safely store user data

Yes it is possible , since your browser will show each call done by your app, using chrome you could openthe chrome dev tools and have a look at the network tab.