OK, I got a clearer explanation from a moderator at Microsoft Learn:
"Thank you for your response, we understand how frustrating this situation must be for you.
For your inquiries, we are glad to provide explanations for your questions in this section.
Microsoft Entra ID (former Azure Active Directory or AAD) is Microsoft's cloud-based identity and access management service. It allows you to manage identities and access within your organization. The Entra ID tenant helps you create and manage amount of user accounts and application access to Azure services, ensuring that everyone in your organization has the appropriate access they need.
You might likely have a Microsoft Entra ID tenant maybe in the past you might sign up for a Microsoft account that relevant to AAD, such as work or school account
What's the difference between a personal Microsoft account and a work or school account? | Microsoft Community Hub

When you sign up for these services, a tenant is automatically created for you. You can read it via this articleDemystifying Microsoft Entra ID, Tenants and Azure Subscriptions | Microsoft Community Hub
Please note that this emaildoes not affect your data or personal account*. If no purchase is made by the deadline, your Entra ID tenant connecting to your business subscription (this has been inactive for a long time) will be disabled - however, it has no effect on your current personal Microsoft account and subscription you are having.*
We hope this information is helpful."
Of course, I don't have an organization, work, or school account, and have never had one, nor have I knowingly signed up to a business service, so it's still a mystery why I received the email. It's only natural to worry that one's account may have been compromised...