You can create various polygons in your project with the Polygon tool in the Sketch menu. There are four default polygons you can create:
Once you select the Polygon tool, you can select the polygon type you want, select the grid to place its center point, and drag. You can always change the number of sides, the length of each side, and the radius by selecting the dimension labels and defining the values.
The sides of the polygon are connected, so you can scale the polygon and therefore change its radius and side lengths by selecting one side and pulling it. The entire polygon will adjust.
However, if you want to handle each side separately, you can detach the sides from each other with the Disconnect constraint. Select the sides you want to disconnect, and then Disconnect. Notice that the shared points are no longer filled.
Watch this video to see how polygons are managed: