new or edit background shapes

Is there a way to get other shapes, circles, etc.? If I made one in another program, what would I be able to do with it?

There aren’t any additional shapes, by design. It’s not something that has been ruled out for all time, but especially for version 1, the idea was to provide a very simple toolset that had expressive capabilities built into and otherwise text-centric approach. The note model serves the text within it, as a way of making text less linear. It comes at this type of software from a slightly different trajectory than a typical diagram-based program, where having stuff like shape libraries and a bunch of appearance options is part of the focus of the design.

The only thing I can think of would be to create an image of a shape and then import that, but it would just be a picture of a shape—like a photograph, it would get blurry if you made it too large, and you wouldn’t be able to type within it.