All the different versions of the game were available with the same tracks and the same in-game track editor. 6 tracks were originally proposed with the game : 5 of them were named upon 5 of the 6 game opponents and one was the default track. The opponents with no "home" track is Otto.
In order to be valid, a track in Stunts must be "round", that is that start line and finish line have to be the same place and that track can't be discontinued. Also, the game engine has numerous limitations that force you to create "real-looking" tracks and then doesn't allow to create any illusion track. To do so, you will need an external track editor.
Apart from that, even with in-game track editor, you can do some crazy tracks by simply creating a round track around the start/finish line, and then create your "real" track with Stunts elements added without considering the reality of their order. For example, if you add a banked road, the game force you to add at least a beginning and an end of the banked road. By using a fake small round track, you can add a banked road without beginning or end. This is due to the fact that the game engine doesn't consider elements outside the round track as track elements, it considers them as scenery elements.
To create tracks, you will need to use a track editor, either in-game track editor or one of the external track/terrain editors.