Hey there, Merja Mikkonen
What Microsoft 365 package have you purchased?
In terms of being asked to purchase Entra ID. This is a separate package that allows you to set up user accounts for business organisations to allow them to access resources such as Microsoft 365, Azure and others. You can find out more about this here: https://learn.microsoft.com/en-us/entra/fundamentals/whatis
If it is only, you who has purchased a Microsoft 365 business package, they allow you to have one business account with a custom domain. However, not all packages contain Microsoft Entra ID. You can find out more about what is included in these packages here: https://www.microsoft.com/en-GB/microsoft-365/business/compare-all-microsoft-365-business-products-b
Hope this information helps,
Nathan