Basically, you write Javascript. The Javascript communicates with native components (Java on Android, Objective C on iOS, C# on Windows).
The communication occurs through the so-called “bridge”. If at any time you feel that this communication slows things down too much, you can choose to implement the Javascript functionality in Java, Objective C, or C# respectively in order to run purely native. In this case, you are writing directly in native code, so there’s no Javascript to native compilation.
This will sacrifice compatibility for performance. Normally, this is not necessary.
- Further reading
Understanding React Native bridge concept