How to install split-apk as system app? I would like to know if its possible to convert a split-apk user app to a system app. One example is the google messaging app, it has a couple of apk files, lib and oat folders and base.dm file in its own /data/app/ folder. I tried copying everything to /system/app without success.

This question has already been asked here, but it hasn't been answered.

I'm using Android 10 rooted with Magisk. I already tried moving a regular .apk file to /system/app without any issues.
Figured it out after a bit of troubleshooting. Done using Root Explorer.

Copy "base.apk" and "lib" folder from "/data/app/xyz" to "/system/app/xyz" and set the permissions. Make  sure to not delete the files from /data/app/xyz after copying!

Example with Google Messages:
- Create a folder called "GoogleMessages" in "/system/app"
- Remount "/system" as read-write when warned
- Copy "base.apk" and "lib" folder from "/data/app/com.google.android.apps.messaging-xyz" to the new folder
- Rename "base.apk" to "GoogleMessages.apk"
- Give respective permissions to the folders and files.

Permissions for folders -> rwxr-xr-x
Permissions for .apk and files inside lib -> rw-r--r--

I can't guarantee that this will work on every device, just make sure to have a proper backup ready if something breaks or just use the file manager in recovery to delete the created folder.