I cant seem to update my 2015 IMac past Monterey 12.7.6 as the os

That sounds about right >> Monterey macOS 12.7.6

macOS Ventura is compatible with these computers

macOS Sonoma is compatible with these computers

Explore the Apple Trade-In  Process where the Value of the exiting device could be used against the cost of a new Apple Computer, iPhone or iPad 

Notation - Some devices and depending on the computers age may have little to no value at Trade In time.

But at least the device will not arrive to the Land Fill Site and be recycled in a responsible way by Apple

Or Shop Refurbished Apple computers at an Apple Reduced Cost

Have used both previously and have been satisfied each time.

Gideataff wrote:

Thank you, I guess a 2015 model is considered obsolete now

If that Mac is still doing the job that you need it to do, you could keep using it whether it is "obsolete" or not.

That machine has a very nice 27" 5K Retina screen, has user-upgradable RAM, can accept lots of RAM, and has connectivity that is still pretty decent (other than the absence of any modern form of Thunderbolt).

Since this is an Intel-based Mac, there is – or was – the option to go to the "Dark Side".

Install Windows 10 on your Mac with Boot Camp Assistant - Apple Support

(Some sites suggest that you can no longer purchase Windows 10 per se, but that you can download Windows 10 from the Microsoft site, and then activate it with a purchased Windows 11 key.)

If you upgrade, you might know of some friend, relative, or charity who would appreciate the machine - even given its age and limitations. As long as you're up-front about things – and prepare the Mac properly for a new home.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

That sounds right.

Big Sur is the end of the line for the iMac (Retina 5K, 27-inch, Mid 2015).

Monterey is the end of the line for the iMac (Retina 5K, 27-inch, Late 2015).

So your 27" 2015 iMac must be the Late 2015 model.

Monterey is no longer one of the "most recent three", so vendors like Adobe and Microsoft will soon be abandoning support for it – if they have not done so already.

It is good enough to run the current versions of Firefox, LibreOffice, the GNU Image Manipulation Program, and the commercial Affinity V2 applications (Photo, Designer, Publisher).