Hello ! I'm Aymeric, an independent advisor, and I'm here to help !
If Windows 10 is activated with a valid digital license or product key, upgrading to Windows 11 will carry over the activation and the “Activate Windows” watermark will not appear. However, if your Windows 10 is not activated, the watermark will remain after the upgrade. To remove it, you'll have a buy a Windows license.
Please let me know if you have any other question !