Let's assume that you have created some sheet music in Noteflight (or any other notation software that allows MusicXML export) and the music has several instruments in it. In case when the layout of the music is simple enough you can upload the XML to Match My Sound Teacher Tool directly. Then the parts will be generated automatically. This is what happens in the video. However, if you need more control over the looks of the parts then you should open the exported MusicXML in MuseScore and create parts there. Then Match My Sound will use the existing parts instead of generating new ones.
Uploading a multipart scores will also automatically create a minus one backing track. So for example, when you have a four-part choir piece then the sopranos can practice with altos, tenors, and basses in their earphones. For now this is the only way of creating backing tracks for a teacher.