Affinity Designer's Contour Tool can be found in the toolbox. Click on that and you will see the options will appear along the top
It creates a basic contour at a certain distance from the original design either positive or negative in the same color and stroke. It can be used with normal paths as well as text and also open paths.
It has a number of options which can be found in the control bar along the top of the app. The options include the radius as well as different types and the fill / stroke as caps and also fills. Some are useful for certain types of shapes so the caps are of no use if you have a closed path but are super useful if you use an open path. They are all live so once you have added the effect, you can go back to the path at any time and change the type, fill, radius etc.
Select a path or multiple paths and then apply it such as dragging it in or out or using a bevel or miter or rounded type etc and then go off and work with another part of the application. You can always come back to the effect and modify it either interactively or via the options along the top. This is the case until you bake the design. If you are using a live star or live donut etc they are also still live until you bake the design. It should be noted that there seems to be an issue with the interactive shape settings if you have multiple shapes, it only works with one shape at a time.
There is an option on the top right of the screen in the options, 'bake appearance'. Click that to freeze or convert the design to a curve and then you can continue to add a new effect to that and so on.
Select a path and if curves (convert to curves if not) and then you add the effect to it. You can still modify the nodes and change the overall shape of the path and the design will follow that, it is still live. As you drag or add or remove nodes, you will see the effect change and sometimes a subtle change in the radius (either interactive or via the control bar) will reveal new shapes and forms in that artwork.
Sadly there is no feature where you can create multiple entries, it is only a single radius, I would love to see a copies added to that and for them to be multi-colored. You can create this by create multiple copies of the artwork (if it is single shape, this is not going to work for a complex vector artwork) and then select each in turn and set the radius to a different value for each with the largest radius for the bottom shape. Change the color for each curve.
You can create a standard path via a shape or the pen (closed) and apply the offset effect. You can create a quick open path with the pen and use the effect on that and create a fillable object
Rounded joins, Miter Joins and Bevel joins - the corner points of the curved object. There is a miter setting as well for use as a threshold for the miter join. The bevel join cuts off the corners as a straight line. The end result of these types depends on the underlying shape so they will work differently for a square to a circle (no effect) and to a star.
This has three settings: auto closed, forced open and forced close. If you have a basic closed path, the forced open will create a nice frame based on the shape as well as the current radius setting. If you are using an open path, the forced open will have no effect. The forced close works well with open paths.
Go to any shape (such as a circle), select and go to the control bar and to the fill and set to force open and then apply by dragging outwards to create a quick ring design. Click the bake appearance button to turn it back into a regular path where you have a fill / stroke etc and you can then fill the design with additional paths and gradients and more.
Select the pen and then add a node, shift a little to the right and up and add another node and then shift right again and drop back to level with the first point and add another node and so on to create a zigzag open line. Set a fill color to green or blue etc and then apply the effect to that and set the fill mode to auto close and you will then see a filled design with a stroke that is closed. To turn it into a regular shape, click bake appearance. You can now add gradients, shapes etc to the design. Set the type to rounded etc as required. Once baked, you can then manipulate it further with the node etc features.