Did you buy it or via a physical card/email ? Is it pre-installed on a new laptop ? Which Office version (Home & Student, Professional, etc.)
In older editions of Microsoft Office (such as Office 2010 or 2013), the full 25-character product key was stored directly in the Windows Registry. Third-party "key finder" programs could easily scan your registry and display the code.
Here is a comprehensive guide to understanding, choosing, and safely using an MS Office key finder for the 2021 edition. How Office 2021 Key Finders Work
It sounds like you're referring to tools—software designed to retrieve lost or hidden product keys for Microsoft Office 2021 from your registry or system.
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Using these tools carries severe risks that far outweigh any short-term financial savings.
Search your email archives (including spam and trash folders) for the confirmation email from the retailer where you bought Office 2021. The subject line may contain phrases like "Your Microsoft Office product key", "Order Confirmation", or "Your software is ready". This is often where the product key is initially delivered.