Transparency can be applied in lots of ways in Affinity Designer.
Here are the typical methods for its use.
Opacity is a term you'll see in Affinity Designer—this is the opposite of transparency. An object with 0% transparency is fully opaque (100% opacity), while 100% transparency means the object is invisible (0% opacity).
You can use the numerical keys to set the layer opacity of a selected layer(s) or object(s).
This quick access to opacity settings also applies to brush tools in Pixel Persona.