I have a 15inch Mid 2012 version MAC on the 10.13.16 MacOS High Sierra. I need MacOS 10.15 for Microsoft downloads

You can get macOS 10.15.7 Catalina from this page. Be aware Catalina and above runs 64bit apps only.

How to get old versions of macOS - Apple Support

MacOS Catalina 10.15.7 - Technical Specifications

General Requirements

OS X 10.9 or later

4GB of memory

12.5GB of available storage (OS X El Capitan 10.11.5 or later)*

Some features require an Apple ID; terms apply.

Some features require a compatible internet service provider; fees may apply.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Catalina:

MacBook (Early 2015 or newer)

MacBook Air (Mid 2012 or newer)

MacBook Pro (Mid 2012 or newer)

Mac mini (Late 2012 or newer)

iMac (Late 2012 or newer)

iMac Pro (2017)

Mac Pro (Late 2013 or newer)

How to get old versions of macOS - Apple Support

Don't use FireFox for these links…

https://apps.apple.com/gb/app/macos-catalina/id1466841314?ls=1&mt=12

How to upgrade to macOS Catalina – Apple Support

And caution on 10.15 or higher, to see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/

Or, Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

https://www.libreoffice.org/

BDAqua wrote:

Or, Look into the Free LibreOffice, it handles MS Office files like Word & Excel, & Page Layout…

FWIW, although LibreOffice is a great alternative to Microsoft Office, it does not include an alternative to Microsoft OneDrive.