That depends on how you downloaded them, George, whether it was in your browser or from inside Affinity Photo. IMO, the best way of doing it is from within AF itself as you can keep track of your free or purchased assets all in one place and also see if they're installed or not. If you did download them with your browser, open a Finder window and Search for ".afbrushes" as this is the file type you would have downloaded. If you can't find them, look in the program itself.
In AF, click (or tap) on the your Account icon (on both the iPad and the Mac, it's the little green man at the top right) and you should see a list on the right of everything you bought or got free. Against each item you'll see a Download icon or a green Check icon. I think you know what to do from here.
You might find this page useful for the future:
A guide to installing creative resources purchased from the Affinity Store
If you’ve bought add-ons from the Affinity Store and want to know how to install them into your Affinity software, you’ve come to the right place.
affinity.serif.com
Edit
There is one other thing you might need to know. When I installed the free overlays packs, they didn't show up in the Assets Studio. I tried to 'Import Category' but couldn't find any .afassets file anywhere on my iPad or my iCloud Drive. So I used 'Add Category', which added one called 'Unnamed' and then all of my assets showed up and were usable. I just renamed 'Unnamed' to 'Garry' in case the others disappeared again. (Superstitious? Not me!
)