Hi PewPewUGonnaDie,
I'm Dyari. Thanks for reaching out. I will be happy to assist you in this regard.
1-Actvating Windows on your new build PC depends on what type of license did you have on your laptop. If your laptop came preinstalled with an OEM license, the license was tied with your old motherboard and it would not be transferred. If it was a retail key try to reactivate it:
Settings > Update & Security > Activation > Change Product Key and enter the key that you have.
If it is a digital license linked to your Microsoft account, it should activate the Windows automatically after signing in with your Microsoft account.
If did not work, try to troubleshoot it by going to Start > Updated & Security > Activation > Troubleshoot.
Get help with Windows activation errors:
https://support.microsoft.com/en-us/windows/get...
2-If you don't have a digital license or a product key, you can purchase a Windows 10 digital license after installation finishes. Here's how:
A-Select the Start button.
B-Select Settings > Update & Security > Activation .
C-Select Go to Store.
D-This opens the Microsoft Store app where you can purchase a Windows 10 digital license.
https://support.microsoft.com/en-us/windows/act...
3-If you want to upgrade your Windows to W11, first, activate Windows 10 since it is currently running on your computer, then upgrade it to Windows 11 (No need to buy a new license key. It will use the same W10 license):
https://answers.microsoft.com/en-us/windows/for...
https://answers.microsoft.com/en-us/windows/for...
Please let me know if you need further assistance.
Stay safe