You can find the Affinity Photo Pixelate Filter in the distort category in the filter menu (which is an odd location). The tool is not available for some reason as a live layer effect which is a pity.
The panel appears and has one option, it is not interactive either so is not changed by dragging across the document. Start with 1 and you can barely see the difference but as the number for the quantisation increases the image or layer will get increasingly filled with square blocks.
Set the value to 100 and you will be filled with large blocks of color which is based on the averaging of the color of those pixels 'below'. You can change the color of those blocks by changing the color of the image before you use the effect or you can modify the color of the blocks / squares afterwards by using adjustments.
You can always apply an adjustment layer to modify the color of the generated square blocks. Go to the layer menu and then new adjustment layer and then add vibrance or HSL or a color LUT or perhaps go the view menu and studio and select the adjustments panel and select one of the presets to apply to the squares. you can then modify the color of the effect.
You don't have to apply the effect to the entire layer or image, you can apply it to a different smaller layer. You can also apply duplicate the image and apply different effect quantisations to each layer and then combine the layers using blending modes via the layers panel and create a far more complex design than a basic effect.
You can use the various selection tools to select one of the blocks and then fill that with say an image or a gradient etc. You can also use a selection to select part of the image and then apply the effect with one setting of quantisation and then select a different part of the image and apply another quantisation setting to create an unusual combination of blocks across the image and perhaps combine those selected areas with different adjustment layers to create unusual colorful block designs.
You don't have to apply the effect and quantisation settings on their own, you can also combine them with other effects such as applying the Gaussian blur before or perhaps applying it afterwards. You can also use the PF with procedure textures with the equations to transform it or perhaps the mirror for some unusual square mirrored designs. You can also save all the steps using these combination of effects into a macro via the macro panel and then saving it to the library for future use.
You can fade the effect, this does not alter the quantisation applied but you can blend the image and effect using blending modes and opacity and you can find this command via the layer menu. You can then the block design to a blend such as difference or darken etc
You can also use the effect with channels. Find the channels panel in the view menu and studio and then select the red channel and apply the effect with a quantisation value of 10 and then go to the green channel and apply it with 20 and so on. The end result depends on the image. You can experiment with the quantisation settings as well as the combination of others to a particular channel. It should be noted that the merged channels RGB may bot be square any more as the alignments of all the squares will vary depending on the settings - great way to create a super colorful design from even the most basic of images
The effect works well with the pattern layers in the application. Select a layer and then go to the layer menu and new pattern layer from selection and set the mirror option on with the move tool selected. Apply the effect and then re-size the pattern layer. The effect is super useful if you modify the size and angle of the pattern layer and then re-apply the effect and you can repeat this over and over to create all kinds of unique layer effects using the same settings as well as varying the level of quantisation. You can also select a circle made up of a basic gradient and go to the layer menu and rasterize (it needs to be a pixel) and then go to the layer menu and create pattern layer from selection. Re-size and rotate as required. Go to the layer menu and rasterize and then go to the menu command and vary the quantisation to create many variants of the square blocks (pity there is no vertical and horizontal setting)
As with many of the tools in the application, you can also repeat the effect and straight away with the pattern layer, you get another amazing pattern design, though it should be noted that repeat applies can sometimes end up just losing all of the artwork especially with this effect but if you combine it with other effects and then use the tiles then you can create zillions of amazing seamless tiles.
You can make a layer via the new layer command and then fill it with black via the fill command in the edit menu. Go to the noise category and add noise and set to the maximum of 100% and then goto the repeat command in the same menu and apply that two or three times to fill it with intense colorful noise. Go to the same menu and run the effect and set it to about 40 or so and this will result in an image filled with random color squares. It is a layer, you can duplicate the layer, you can combine with others using blends etc
Open an image or perhaps go to the tools panel and select the ellipse tool and create a circle and fill with a colorful gradient made of blue and reds and greens etc. Resize the circle as required and go to the layer menu and rasterize. Go to the new pattern layer from selection in the layer menu and then create a pattern design and delete the source. You can then resize and rotate the pattern but you can also go to the effect and set a quantisation value of 400, 401 etc and as you change the value you will see the colors of the squares change in the pattern design. Once you select something you like out of the 100s of variations, go to the layer menu and merge visible and make a pixel layer of the whole design. You can now go to another command such as deform or spherical and distort the artwork