You can find the Affinity Designer corner tool in the tool bar near the top of the panel
You can modify the roundness of the shapes using the move with some shapes such as rectangles as well as cog designs and stars and more. Go to the top of the application and the options and set the value as well as the type such as rounded and straight etc
Use it to modify the roundness of the design. You can select one of the nodes (click the little square box as the node and you will notice that the square is now filled) or multiple nodes and change interactively the radius by dragging the small red dot. You can also change the radius via the slider in the options along the top of the application. You can set the value to a certain maximum value. The radius will not go beyond the maximum curve for the path. There is a limitation that if you select multiple nodes, the radius will stop when any of the radius settings matches the maximum. If you want to change another radius further you must deselect and work on the nodes that you can change.
By default, there is no rounding with text and it is set to none. Likewise with circles. The key thing is that there is a sharp point to begin with. A range of types are available : none / rounded / straight / concave / cutout.
You can reset the design back to the sharp point by double clicking the interactive red spot or filled square spot. You can then re-drag the interactive control outwards to the maximum value.
You can click the bake appearance button along the top of the options to lock / freeze or bake. Once this has been done, you will probably (unless you still have some sharp points to change) still be able to use it again and again. You will see the result as tiny little dots on the entire path, you can then manipulate the nodes in a number of ways to create all kinds of unique designs from the curved path.
You can select multiple paths at the same time and modify them but they all need to be the same type of shape, you cannot round a rectangle with an open zigzag path etc but if you have 10 stars selected then you can use it to modify those ten stars at the same time even though the control points will show that only one is active but all will change.
Go to the pen and add a node to the art board and repeat and close the path. Once you have done this, select and change the type to rounded and click on the first point and drag toward the center of the shape and you will see the control point (and the control circle) and release. Go to the other nodes and repeat and you will have a curved triangle design. You can also select all the nodes and then apply the effect by dragging and the curvature will be applied to the maximum possible setting. You can drag them to the point where it becomes a circle. It should be noted that the outlines generated when you push the shape to the max sometimes (but not always) matches the end result. You can also enter a radius setting up to a maximum of 1000px
You can also use this with open paths such as creating a very quick zigzag using the pen. Click on the art board to create multiple nodes in pen mode. Select all the nodes in this path / curve and then drag with the effect and you will see all the control points change at the same time. If you stop at any point, you can always re-click one of the on screen markers and re-start the drag. You can also enter a value up to 1000px in the radius entry. If you use the interactive controls you may find they stop working well before the 1000px but you generally can extend the curve beyond. Of course, the best looking curve maybe at 50px or 100px etc or you may prefer to tweak each of the nodes individually. Once you are happy with your design, click the bake appearance button.
With text, select and then right click and select the convert to curves command. Once you have this, Double click on each character to display the nodes and then go to the square sharp points on the text and apply the effect by dragging the control point (or changing the radius in the control bar). You can select all the control points and drag them all simultaneously.