Hi all.
Quick
PSA.
To explain what actually happens when you install ReVanced using the manager.
Note: the file path might be different if you use the extended repo.
I have not tried yet but, I think the extended repo used /data/adb/
instead of /data/local/tmp/
When you open the manager you see the installed apps that are suported for patching.
When you select one (YouTube for this example) for patching, the manager copies the stock installed apk file to a working directory.
The copy is then patched with the patches you select and a patched apk file is created.
When you choose to install it, the patched apk file is moved to
/data/local/tmp/revanced-manager/com.google.android.youtube/
.
The permissions of the modified apk file needs to be set correctly.
This can not be done in a normal user level like /sdcard/
so /data/local/tmp/
is used since it has heighten permission levels.
Two boot scripts are created, and placed in Magisk's boot script directories.
One placed in post-fs-data.d (early boot) and the other placed in service.d (late start).
This is different than a module.
A module (if scripts are needed), include their post-fs and service script within it's own module directory instead of the main directories.
Then the
mount
command is used to overlay the patched version on top of the stock installed version.
When you open the app, Android is pointed to the patched version instead of the installed version.
Since the mount overlay will not survive a reboot, the boot scripts that were created (post-fs and service) run the mount overlay on boot.
Otherwise you would have to mount the patched version over the stock version manually after every boot.
Magisk's Overlay does not extent out to user data. That is why we use the mount command to overlay the modified apk.
---
There is still an issue with ReVanced not replacing the previous patched apk file and/or scripts when you update or re-patch an app.
In a previous post I gave instructions on how to clear ReVanced YouTube for updating.
Post #
182
Still an open issue.
ReVancedManager - [ GitHub ] - Issue #
425
I happened to comment in the above issue what and why we use boot scripts and the mount command. 
---
Hope it helps more than confuse.
Cheers.