That depends on how the key was purchased,
If you purchased it directly from the MS website then the product key is in the confirmation email you received after buying your digital copy of Windows. Microsoft only keeps a record of product keys if you purchased from the Microsoft online store. You can find out if you purchased from Microsoft in your Microsoft account Order history.
If you bought Windows 10 or Windows 11 Pro upgrade in the Microsoft Store app, you'll receive a digital license instead of a product key in the confirmation email that was sent to confirm the purchase. That email address (MSA) will contain the digital license. You can use the digital license for activation.
And you did double-check your email Spam/Junk folder? If you did search everywhere in your email and nothing was sent at all to you, then I suggest getting in touch with a MS Service Rep. You can send the request in the link below.
https://support.microsoft.com/ko-kr/supportrequ...
Kind Regards,