There are basically two kinds of music sources you can use. They can be on the Internet or locally stored and accessible through your Wi-Fi network.
First of all, Spotify – the world leading music streaming service. Use the Spotify app and stream through the network music player into your speakers. The Spotify streaming service is integrated in the player unit and the music is actually directly streamed from Spotify's servers to your phone or tablet - leaving your phone free for making calls and all other uses. You only use your phone to control music playback. You can also just pick up the music where you left off when switching between your phone, tablet or computer. It is seamless streaming.
Then if you play music or listen to radio using an app on your smart phone or tablet, you can redirect the stream to the network music player and continue listening. You can actually stream any audio to the network music player. Like music stored on your devices or podcasts, music players, radio apps or other streaming services than Spotify as Pandora, Qobuz, Tidal, Deezer, iTunes etc. Any audio stream can be used.
You might also have digital music files that you downloaded yourself or ripped from your CD collection, often stored on devices such as a PC or Mac. You can even use special dedicated network attached file storage if you have a really large digital music library. As long as the devices are on the same Wi-Fi as the network music player you can stream the music through the player using the local software accessed through player.local in any web browser or use the special player apps.
The only source that you might attach directly to the player is a USB flash drive.