To create amazing new Affinity Photo brushes from selections, a great point is a shape but equally images. Shapes cannot be used without rasterising them, they need to be a pixel layer. Go to the shape tools such as the star tool, donut tool, etc. Create your shape design, set color etc as required. To rasterise the shape, go to the layer menu and rasterise. You now have a pixel layer which can be picked and used a source for amazing strokes. Go to the select menu and then selection from layer or just have it picking it is fine or use the rectangular marquee tool etc for part of the design. You have a raster design which can now be converted into a new preset. You can also add effects and stroke designs etc to it if you wish.
Go to the brushes panel, this can be found via the view menu and studio and then go to the right side menu and then new brush from command. You will see your design added to the panel.
To modify the stroke further, double click the entry and then change the size, dynamics etc as required. You can modify the hue, saturation etc and have the stroke change colors etc as you apply it.
You can use patterns as a great source for presets. Choose a shape or another design and then use the pattern layer from selection feature found in the layer menu. You can then re-size the pattern and rotate etc but to use it as a stroke preset you will need to go to the layer menu and rasterise the pattern layer to a pixel layer and then that can be defined as a preset as described. You don't need to rasterize, you can also just add a rectangular marquee tool etc over the pattern and as long as part of the bounding box has been included in that, you will be able to save that as a preset also. The preset can then be modified in countless ways. You can also create multiple pattern layers and combine them in infinite ways and use those as sources as well for presets.
You can also use gradients as a great source for presets. Create a new pixel layer and then go to the gradient tool and add your gradient, you can then modify the gradient by adding stops and colors etc. You can also set the 100% setting to 0% opacity to create a transparency for the rest of the layer. The design can then be added as a preset via the new command
You can also use strokes themselves using all the tools available in the application such as paint tool, blur, mixer etc and apply those to create all kinds of amazing designs as well as mix in with some effects or two such as deform and then use that as a source design via the same technique
You can also just use any photo / design as well as a source and modify with effects such as blurs and deforms and then use that as a preset design. You can also use the rectangular marquee tool or elliptical marquee tool etc and then use that to create a new preset via the right side menu command.