The denoise filter for Affinity Photo can be found in the noise category in the filters menu. Removal Reduction Smoothing
The effect has a number of options to manipulate the noise in the image. The panel is split into two sets, the luminance and noise as well as the color and noise. If the image has a lot of color grain then you can use the color settings at the max and keep the luminance ones low, or vica versa.
You can use it with images as well as type and also presets such as brushes and shapes / shape tools and fonts / EPS shape sets and patterns and much more. The result depends on how much noise there is with the type or shape and as they maybe added as a solid color there may be little need for the smoothing effects.
You can store a set of steps by recording and this is done in the macro panel (view menu) and you can combine the effect along with others and the macro can be re-applied over and over to create amazing designs including repeat applies of the effect. The effect ends up becoming quite extreme and surreal if applied 20 or 30 times to any image and the macro feature really makes it very easy to apply the effect over and over again without the need for shortcuts etc
You can use it on a layer (with a shape or image or text etc) and the effect can be applied and the layer can then be quickly blended with the background layer using a whole range of blending modes. Also, layers are super useful as if the effect doesn't look great later on then you can always just remove the layer and start again without any destructive effect on the image.
You can also use the feature with a noisy image in a particular channel. Say the image you have is very noisy in the red and not much in any of the other channels. Go to the view menu and studio and channels and select that channel, say the red channel, and then apply the color and luminance settings to that channel alone and then reset back to the full RGB etc
You don't have to apply the effect to the entire image, you can use selections such as the freehand selection and select say some lips or eyes etc if they are particularly noisy and then apply the luminance and color contributions depending on the image. You can also then go to another selection and then apply the effect again to that.
Hardly see much of an effect if applied again, the removal works fairly well, certainly on my images. Of course, there are zillions of different images and perhaps the settings will require more and the image improves after a few applies (must admit, never have that much problem with noise as I always loved the noisy photos that appear in the 60s magazines in B&W etc)
Go to the layer menu and select fade. I love the fade as it means you can change how the original and effect interact and as with all the effects, some blends are excellent such as the hard mix, negation, linear burn etc etc if you are look for visual imagery