The viewport is where you will arrange the various sources in a scene.
You can drag and drop some sources like media files directly on to the viewport. Other sources have to be added via the “Add source” option.
Once you add a source, you can arrange it on the viewport however you like. You can resize a source and move it on a 3D axis by holding down the shift key while adjusting a source. You can also do precise movement of sources by using the arrow keys.