Windows
Windows. That is the whole list.
V2 is a Windows app. Store Fortnite and Epic Fortnite live in different folders — pick the one you actually play.
Epic path
Usually Program Files\Epic Games\Fortnite. Custom libraries follow the drive you picked in Epic settings.
Microsoft Store Fortnite
Sandbox path. If paks are invisible, you are in the Store package, not the tree the swapper expects.
Controlled folder access
Ransomware protection can silently block writes into Program Files. Allow the app or move the library.
Admin?
Only if writes fail in a locked Program Files tree.
Windows 10?
Yes if Fortnite still runs there.