Data Creation Tool and a Clean Install

About a week ago, I had to do a factory reinstall using SupportAssist.  Dell techs worked 1 1/2 hours to try to fix.  It didn't go well, but I eventually got things fixed myself.  I was not at all impressed with SupportAssist, and if I ever have to do an install of the OS, I will use Microsoft's Media Creation Tool and do a clean install:

https://www.microsoft.com/software-download/windows11

The install would  be on this same machine, on the SSD.

I have two questions:

1) What's the difference between the 2nd and 3rd methods?

2) I don't understand how Microsoft knows the Product Key of my OEM install.  How is this conveyed?  Can I find it out?

Thanks.