• APK file: Tap on the downloaded APK file, and follow the on-screen prompts to complete the installation.
• XAPK file: It's a compressed package containing an APK file and additional data files. You need a XAPK installer to install it. If you're using MEmu Android Emulator on PC, simply install the XAPK file directly into MEmu.

Yes, you can use UnderDark : Defense on your PC using MEmu Android Emulator. After installing MEmu, you can either drag and drop the downloaded APK/XAPK file into the emulator, or open MEmu and search for UnderDark : Defense to install it directly.