What are the two Todoist mapping modes and how do I choose?

In the Todoist card you pick one: "One project" (the default) puts everything into a single Todoist project with a section for each Supernote list — tidy if you want it all in one place. "A project per list" gives each Supernote list its own Todoist project. You can also set the default project name. Either way, the app finds or creates the right project/section for you automatically when you push.