XAPK will not install

XAPK, APKS, and a split APK are a package in several pieces. The phone's ordinary installer will not open that as one APK, so the XAPK will not install.

Renaming it to .apk does not work. Inside are the main package and extra pieces. Samsung My Files and the Xiaomi file manager can open the archive and install one piece. The app opens and closes at once.

• Changing .xapk or .apks to .apk does not fix the package. The phone still sees one piece.
• Install every part together with an installer that joins the pieces. If you install only one piece, the app opens and closes.
• Samsung My Files and the Xiaomi file manager do not install the archive themselves. They open the file. Installation is another app's job.
• If an OBB or data folder sits inside a game, the APK alone will not open the game. That folder has to go to its own place too.
• Play itself sends split packages. An XAPK from outside is those pieces gathered into one file.