Split APKs Installer is an advanced APK installer with split APK support. It also has simple app backup/export functionality.
SAI can install apps using standard Android API, using root access or using Shizuku.
SAI is an open-source app, you can get the source code on its GitHub repo – github.com/Aefyr/SAI
Please note that SAI only installs user-provided files and is not responsible for those apps behavior.
Please also note, that some devices/ROMs aren’t supported by SAI, that has to do with bad Android API implementation in said ROMs and can’t be fixed on app side.