The star in the left toolbar is a shape library. There are many prebuilt shapes there. You can draw rects and circles by clicking on the corresponding tool and

The star in the left toolbar is a shape library. There are many prebuilt shapes there. You can draw rects and circles by clicking on the corresponding tool and clicking and dragging on the canvas (standard drawing app conventions). You scale and resize the intance by dragging its corner. If you want to draw perfect circles and squares hold shift while drawing.