XAPK Files on Android
XAPK bundles an app's main APK together with its OBB expansion data (or split config APKs) in one .xapk file -- it needs a dedicated installer app, since Android has no built-in XAPK support.
1Understand what's actually inside an XAPK
An .xapk file is a zip archive containing either (a) one APK plus an OBB data folder for a large game's assets, or (b) a base APK plus several split config APKs (per-architecture/per-language) the way the Play Store itself delivers big apps. Either way, Android can't install the .xapk file directly -- it needs to be unpacked and installed correctly.
2Install a dedicated XAPK/split-APK installer app
SAI (Split APKs Installer) is a well-known, open-source option that handles both OBB-style and split-APK-style XAPK files automatically -- mirrored and scanned on MirrorNest itself.
3Open the .xapk file with the installer app
From your file manager, long-press the .xapk file and choose to open it with SAI (or your chosen installer) instead of the default handler -- it reads the archive and installs the base APK plus its split/OBB data in the correct place automatically.
4Confirm the install prompts
You'll see the normal Android permission/install confirmation, same as a plain APK -- approve it to finish.