The nymea:app is the official community client for the nymea server.
The source code of the client can be found here:
You can download the app from the google play store.
You can download the app for your iOS from the Apple store.
There is a Windows install installer available which allowes you to install nymea-app in your Windows environment.
If you want to use the nymea-app on macOS, you can download the latest bundle form here.