Is React-Native a single threaded execution or multi-threading execution?

It's single threaded. Please refer this link. I think it will clear all your doubts. https://react-native.canny.io/feature-requests/p/parallel-multithreading--workers

Javascript is single threaded. Since React Native is essentially JavaScript (+ native UI), it is single threaded out of the box.

You can however start a new JS process (usually to carry out CPU intensive tasks) using react-native-threads which essentially is a different thread. You can go through the library docs if you want to know more about how it works.