If I know the windows 11 product key in my current version of WIN11 can I not just transfer this to the new MOBO and the reinstalled Win11 on this MOBO. Is this possible?
Since you have a home built PC, you may have a transferrable Retail License, if you purchased it direct from Microsoft or from Amazon direct (not a reseller). A Retail License tends to be the most expensive type (roughly $100 for Windows Home, $200 for Windows Professional). A Retail License allows you to transfer Windows from one computer build to another. You may need to contact Microsoft if they detect a major hardware change when you fit a new mobo and CPU and then access the internet.
Alternatively, you may have bought a cheaper OEM License, intended for system integrators planning to sell on the PC to a customer. To comply with Microsoft's licensing Terms and Conditions, you're not supposed to use an OEM License on your own personal computer, unless you bought the computer from Dell, HP, Lenovo, etc. OEM Licenses are normally "tied" to a single machine and are not transferrable.
If you bought an amazingly cheap key online, chances are you've joined hundreds or thousands of people using the same identical key, split from an Educational License pack (for use in schools, colleges, universities) or a Volume License pack (for business and corporate users, with hundreds or thousands of employees). If you're not a member of the Enterprise or Educational establishment, to whom the multi License pack was originally sold, you're running "knocked off" software and the License is not valid for home use.
https://www.zdnet.com/article/is-your-windows-license-legal-should-you-even-care/
https://www.thewindowsclub.com/check-if-windows-key-is-genuine
If you bought an expensive Retail Pack, you're legit. If you bought an OEM pack by mistake or were duped into buying part of a volume pack, best of luck. You'll probably get Windows installed on the new machine with an OEM or pirated key, but you won't have a legitimate license. I wouldn't recommend contacting Microsoft direct to revalidate the installation, unless you have a Retail Pack.
I don't understand why people happily shell out $15 for a pirated key (and invalid license) when there are nefarious (illegal) ways to install and activate Windows absolutely free. If you're happy knowingly to flout Microsoft's T's and C's, why pay the scammers $15? These cheap mis-sold keys can be revoked at any time by Microsoft, but most people get away with it.