There Isn't Much, Besides One Grand Prix Editor In The Game. Otherwise You'll Basically Have To Use Blender Or An Official Track Maker. Atleast From What I Know.
I think there was one at some point, but I don't know if it works and I never used it. https://supertuxkart.net/Track_Editor Maybe an older user can help with that page.
Most tracks are made using Blender, a 3D graphics tool. There are some guides and a tutorial, but it's not as simple and easy as the SuperTux editor.