Earlier quoted context omitted.
Well, you're running both the iOS development tools (Xcode, iOS Simulator), plus the Android development tools (Gradle, Android emulator, and maybe Android Studio too). These add up. 16GB might be possible, though. (Skip itself doesn't take much memory. If you run it headlessly as a SwiftPM plugin, you wouldn't need nearly that much.)
Do you have to run both at the same time? Because my flow with React Native is to focus on one platform at a time, I don't try to run everything in one shot.
For framework/library development, you can of course build and test separately for each platform.