Android - How to extract stock apk from TWRP backup?

Rename the system.ext4.win file to system.ext4.tarand extract it.
If your TWRP backup is not in compression method, you will directly get to the /system folder or if your TWRP backup is in compression method, you will get a file named system.ext4 which you'll have to just rename it to system.ext4.tar and extract it to /system folder

I discovered that it was not necessary to mount it. It was sufficient to extract it with tar -xf archive_name but this particular app doesn't install.