Hello
It seems like the previous owner of the Dell system had a Microsoft 365 Business subscription associated with the device. If you subscribe to Microsoft 365 Business, you may be able to use your new Microsoft 365 login to access the device and load the OS. However, before doing so, I recommend contacting Dell support to troubleshoot the issue with accessing the BIOS and to ensure that the device is properly transferred to your ownership.
As for your question about the Windows Media Creation Tool, if you use it to install Windows 10 Home on a system that originally had Windows 10 Professional, it will not automatically downgrade your license. The version of Windows that gets activated after installation depends on the product key that is entered during the installation process. If you enter a product key for Windows 10 Professional, it should activate as Windows 10 Professional.
However, if you want to downgrade from Windows 10 Pro to Home without losing data, you can follow these steps:
Change the Registry values.
Download the Windows 10 ISO file from the Windows 10 Media Creation tool.
Perform an InPlace Upgrade.
I hope this helps! If you have any other questions, feel free to ask.