Can I apply a Combo Update DMG onto to an USB installer post createmedia...

You cannot apply the Combo to the installer you already have.

You would need to install macOS Catalina 10.15.1 and then install the Combo update over that.

Or just ditch the installer you already have and download the last version of the installer from the App Store.

Open Safari and click on this link,

How to get old versions of macOS – Apple Support (UK)

Go to the Download OS section and click on macOS Catalina.

This will redirect you to the App Store Catalina download page,

it should say Get or Download.

This will download the last version of Catalina 10.15.7 directly to your Applications folder

as Install macOS Catalina.app, it should be 12.26GBs in size.

Make a bootable USB installer with that.

No, Apple has stopped making standalone updates for macOS Big Sur and any future OS's available for download.

I am not entirely sure what you are trying to do.

If you download the full installers for macOS Catalina or macOS Mojave or any other macOS or Mac OS X

these full installers will install the last version of that OS to be released.

macOS Catalina will be 10.15.7, macOS Mojave will be 10.14.6, macOS High Sierra will be 10.13.6

so there is no need to apply any Combo update to these.

The macOS full installer for Big Sur is 11.5.2 at the moment.

In the context of Big Sur specifically - there are No Combo updates offered by Apple for Big Sur.

So the remainder of the question is mute