We're in the endgame now...
The first step is to either have Xcode installed or Android Studio, depending which one you want to work on. You do not need to edit code in these, you just need them for their emulators.
Then, install the react-native cli by running
npm install -g react-native-cli
Then, to run the application,
cd ./you/know/how/to/do/this/SplitSound-Mobile
react-native run-ios OR react-native run-android
When running either of these for the first time, it will take a long time.
Note: the android emulator has to already be running for the run command to work for android. If you want, you can also connect your phone to your compooter to run it on that. I've yet to find a way to do that on iOS though.
./scripts/android-emulator can be used to start an Android emulator on any platform.
It assumes you already have a copy of the Android SDK installed on your computer and will guide you through setting the ANDROID_HOME environment variable to locate it if necessary.
An existing AVD (i.e. one created in Android Studio) can be used with the script by passing its name as the value for the -a flag.
./scripts/android-emulator -a PixelTest
The name can be retrieved from Android Studio or via avdmanager list avds.