Does Monterrey contain GB App or some other Music making software?

Is GB finished as an App?

GarageBand is very much alive and well. Apple recently updated the Mac application to version 10.4.6 (see the notes here and the security notes here - dated 16 March 2022)

The GarageBand support page at Apple's website is the place to go for the latest information about the app; the page even has a 'What's New' link.

GarageBand is also in the list of built-in Apps when you purchase a new Mac, for example the current iMac.

That is because it is tied to the Mac, not the operating system. It is not part of any OS, but you can download it and use it. Apple is actively working on it.

Yes GB does run on Monterey

I will just say that there were many incarnations of the availability of iLife, including stand-alone separately purchased CD's.