The Affinity Photo rectangular to polar filter can be found in the distort sub menu (which can be found in the filter menu).
It is a fairly basic but still very powerful feature with no options but it can be used more than once as well as combined with others to create some truly unusual effects.
You can use them with brush strokes and fonts and images and layers etc
It is limited if applied once but it can be applied multiple times and each time the effect starts to accumulate and you can end up with some pretty amazing designs.
You can, of course, also combine it with other filters such as mirror and then re-apply. You can duplicate the original source image and create multiple layers and then apply the effect to that and then modify the layer in other ways and then combine the result using blending modes and opacity, though it should be noted that applying it to a layer will distort the layer as well making for unique shapes that can be combined together. You can distort the layers in all kinds of ways, especially slow though if you go for very thing designs.
You can apply it and then use the layer menu and fade command where the effect can be blended as difference or darken or lighten and more as well as the opacity changes so you can see the original image more or less.
You can also use it in selections and you can create a basic selection and then apply it but you can use the freehand selection tool and create a very complex selection by using the add and subtract functionality for the selections and with that the effect is applied to just that selection thought it will still follow the path of the effect. The workaround for that is to add the selection design to a layer by copying and pasting to create a new layer and then applying it to that. As a layer the effect will be applied differently and you can also combine the layers by duplicating the designs and you can also add layer effects and you can also then re-run the effect. You can also use feathering with this as well to create even more unique combinations of the original source image and the curved effect.
It can also be applied to different channels via the channel panel which can be found in the view menu and studio and channels. Select the red channel and then apply the effect and then go to the green channel etc. You can then also go to the blue as well and apply it but with different settings. If you have added some alpha channels etc, you can also apply it to that as well to use it to create amazing and unique selections with this curved effect.
You can also use it to the pattern layer and the effect will be applied to that and create some truly extreme distorted designs. If you do, I would suggest using it with the mirror option set on and that can be found in the top left corner as that means the effect will not result in seams. You can also rasterize any pattern design and then apply the effect to that and then turn that into a pattern layer as well as scale and rotate and shear the design in even more ways.
You can add it to a macro and record the step and use with others such as deform and as well as twirl and blurs and noise effects and others. You can then playback the whole thing at any time. As it has no parameters, you don't need to worry about setting up and macro parameters (this can be with others but this is one of the tools that does not have any parameters or options.)