How to draw onto a shape that I just drew in Preview.app?
In Preview app and once the shape is drawn, you can only move, rotate or resize it.
Here is an article that comes from the Apple's Preview User Guide :

You can add a shape to a PDF or to an image using the Shapes tool in the Markup toolbar, and then rotate the shape or change its sides or angles.

If the hand cursor persists or the shape you just drew does not suit you : click on the Sketch or Draw button again to draw a new shape.
If you want to draw onto a shape you have just added : close the image preview you were editing after saving it and re-open the file so the shape will be considered as part of the image.