All your private materials and Prompto materials can be found under the "library".
This ever-growing library of materials grows in time and you will see new materials pop up every month. When the materials have not been used before you can download them by just clicking on them. After that, they are available for you.
Don't worry we keep our file sizes small, so downloading won't be a long process.
When you want to apply a material to an object you simply click the material and once selected you click the model you want to apply the material to. You will see your model change instantly.