Hey Joe. I work on Exponent.
When you're developing you can use Exponent's apps from the App Store/Play Store, so you can get started immediately and let other people test your app easily. Once you're ready to put your app in the stores you start a build using our CLI and we build it on our servers (see https://docs.getexponent.com/versions/latest/guides/building...). We want to make it easy for people on Windows and Linux to build apps so don't require you to use Xcode or Android Studio. If you don't want the builds to happen on our servers our clients are open source so you can do it yourself if you want (https://github.com/exponentjs/exponent).