Lookupper is available on both Windows and macOS, but it’s not a cross-platform app. The macOS version lacks many features available on Windows, which is why there’s a price difference.
If you buy the Windows lifetime license, you can use Lookupper with Lookupper Pro on up to 10 Windows devices linked to the same Microsoft account.
If you buy the lifetime version from the Mac App Store, you can use Lookupper on one macOS device.