How to install xapk, apks, or multiple-apks via adb? xapk, apks, or multiple-apks is the new standard for apps that is basically one folder and multiple apks in it.

I know there are apps like apkpure or SAI that can do it, but I want to know how to it manually.

I know we can use adb install-multiple to install signed apks but if they are unsigned how can I do it if I have the root?

I used to be able to copy and paste single apk to /system/priv-app/${folder} to bypass the signature check but simply copying and pasting multiple apks under the same folder doesn't seem to work in this case.
For those who are trying to install XAPK files through adb because their XAPK Installer does not work on their Android phone, I've got you covered.
You must have Android Debug Bridge (ADB) set up and working.
XAPK files are basically ZIP Archive files (described by their PK header at the beginning of the file)
This means we are able to open/extract those files using any ZIP tool that we have (I prefer 7-Zip)
Once we have extracted the files from the archive, we are only interested in .APK files.
Open the Command Prompt/Terminal in the folder where you've extracted the archive,
and type the following command: adb install-multiple &quot;AppName.apk&quot; &quot;ConfigA.apk&quot; &quot;ConfigB.apk&quot;
Notes

The order of files in the command does not matter.
AppName, ConfigA and ConfigB are examples, they will differ depending on the package.
There might be more APK files, make sure to include them all.
If there are several APK files with different architectures, make sure to check your phones CPU architecture before proceeding with installing the APK.