You can find the square star tool in the Affinity Photo tools. It is part of the collection of vector features such as the cloud, diamond, ellipse etc As soon as you select the tool, you will see two new options appear in the control bar of the application.
You can set the sides as well as the cutout of the design. You can set them via the top bar and you can also set the cutout interactively via the dot on the shape which can be set between 0 and 100%. So what is this ? Well it is a star design but with flattened points. You can use the sides / cutout to create chunky artworks as well as rounded designs as well as intense burst designs.
You can use the paths as is (without any conversion), you can also convert it to curves (or turn it into a standard path). Why would you want to do that ? You can then manipulate the points or nodes of the design via the node and you can then add points and move points or nodes to create all kinds of magical artwork
You can also keep it selected and then go to the layers panel and go to the bottom and select the effects button and add a shadow or bevel to your artwork.
You can create artwork and then create another shape such as an ellipse and with the two shapes you can select both of them in the layers panel (found via the view menu and studio) and then go to the layer menu and geometry and select add or subtract etc to combine the two shapes to create a new unique vector design.