The Affinity Photo Add Noise filter can be found in the noise category in the filters menu.
The tool only has two options, intensity and type. The intensity setting can be set to 100% though weirdly the intensity can be set to more than 100% but the result looks different. How ? You can use the interactive dragging across the screen and it does increase the value and the result is a lot more grain than the basic 100% setting which is really odd. You can also set it to Gaussian or normal, they are subtly different. You can also have monochromatic (gray) as well if you wish.
You can use it multiple times or once as well as combine them with other filters. Apply the effect multiple times means you can generate a greater grain effect than a single apply. The application can also be recorded and stored as a macro and then logged into the library. You can also vary the settings from apply to apply to create a variety of designs. Apply it multiple times via the repeat feature but then it uses the same settings. You can also repeat it again just by going back to the same menu command and changing the settings, perhaps more or less.
The effect, like all the others in the application, need something to work with so you need a layer and the layer can be anything such as a gradient or photo or pattern design or more. It can be used to apply subtle effect to an image but it can also be used to apply extreme speckled grain to an image. You can also use the effect on a duplicate layer and then use blending modes to combine the effect with the underlying image as well as use opacity. Many different kinds of grain effects can be achieved simply by using two or more layers with the same image and also using blending modes to combine them. You can perhaps vary the settings between the layers as well to create an even more interesting combination of the effect
You can do much the same with the layer menu and fade command where you can use the blending modes and opacity to change the effect. If you apply another effect in between, you will lose the facility to modify and fade the original effect.
The effect can also be applied in different color channels and you can do that by displaying the channels panel (find it in the view menu and studio and channels). Select the red channel and then apply the effect with a particular set of settings. You can then repeat with the green channel but apply it with a different intensity setting etc. You don't have to apply it to all the channels and you can also use the effect multiple times.
Selections can also be used with the effect and you can create all kinds of selections such as freehand ones as well as tonal etc and once you have your selection, apply the command to the current selection. The selection can also be changed and modified and the effect can be applied again.
You can also use it in combo with other such as deform / twirl / perspective where you can distort the effect in all kinds of ways. The deform can be repeated and repeated to stretch the grain in many different ways and you can then combine multiple layers of these deformed layers. You can also use with blurring effects such as lens blur to create really weird effects as well as using it with distortion effects such as lens distortion as well as others. All kinds of variations can be added on the combination of multiple applies as well as deform handles as well as the source material such as using a gradient on a layer. You can stretch the pixels outwards and inwards in all kinds of different ways and use linear constraints rigid or similarity (the latter being the go to effect)
It can also be used with the macro feature which can be found in the view menu and studio and macro and combining with effects such as depth of field blur and others. All kinds of combinations can be used such as repeat applies of the grain and then followed by a Gaussian blur and then a deform filter and then auto level. Once you have recorded everything into the macro, you can save this to the library as well as playback the macro multiple times to create all kinds of unique effects.
You can also use it with pattern layers. You can create a pattern layer from any selection via the layer menu and the source can be the noisy effect and you can also apply the effect to the layer itself. Select a region of the effect and then go to the layer menu and new pattern layer from selection and then go to the move tool and set the mirror option on in the top left and then you can rotate and scale your layer. Multiple layers can be used and the layers can be scaled and rotated and the resulting layer can be blended in countless ways
It is also available in the live filter layers command in the layer menu. You can create more than one layer and the different combination of layers will result in all kinds of unique designs. You can vary the settings of the effect and use blending modes and opacity between these live layers. You can also live edit the effect as it is a non destructive effect. If you want to edit the result at any point, go to the layers panel (view menu and studio) and then double click the entry and you can then change the intensity as well as uniform or Gaussian. You can also then change the blending mode as well as opacity.