Files
Where the looks live
If you cannot find the Fortnite folder, you pointed the app at a screenshot. Browse from Epic settings.
Location
Under your Fortnite install, Content\Paks. Do not copy paks to Desktop “as backup” and then swap the copy.
Verify
Epic checksums those files. Verify = factory cosmetics.
Do not upload
Sharing paks is distributing game files you do not have rights to post.
Which pakchunk?
It changes every patch. Let the app resolve ids.