The rectangular to polar filter in Affinity Photo can be found in the distort sub menu (which can be found in the filter menu). It is a fairly basic but powerful filter with no options but it can be used more than once as well as combined with other filters to create some truly unusual effects. You can use them with brush strokes and fonts and images and layers etc
Filters menu in Affinity Photo
Distort
Rectangular to polar filter
1. Ring patterns for Affinity Photo etc
1. Twirl filter in Affinity Photo and twirling images in many ways