To route channels to these tracks, you can click the direct out selector and choose them, similar to how it works in the track properties in the track inspector.
The Master track is also a group.
This is useful for example for applying reverb and controlling the reverb signal separately.
You can send to these tracks/channels using the Sends section in the track inspector explained in Aux Sends.
Some other DAWs use the term bus to refer to what Zrythm refers to as group. Please keep this in mind if you are coming from other DAWs as they are different things in Zrythm.
Similar to sending signals to FX tracks, you can also send signals to plugin sidechain ports, if the plugin has any. See Aux Sends for details.