Mac Installer format: XAR or DMG?

[ankd1601]

I am working on tool that extracts and views contents of various archives. One such archive is DMG. Tool is correctly extracting DMG archives. But one specific type of DMG file which is macOS Big Sur installer,SharedSupport.dmg or any other similar installer like Ventura and so on, is not getting extracted, as it has XAR header as well. So structure seems different from the other ones. When inspecting the byte stream,I see both a XAR and a DMG structure.

To be more specific, it has:

1. the XAR signature "78 61 72 21" at the beginning of the file,
2. and the DMG signature "6B 6F 6C 79" at the end of the file, i.e., there is a 512 byte-trailer that contains the magic signature "koly".

Any DMG package other than a macOS installer does not have such signature at the beginning of the file.

Does that mean a macOS installer isn't actually a DMG file but a XAR file? Why does it have XAR header?