Document Extensions In OS And In W10 ?

[Robert4]

Hello,

This falls under the category of: "just trying to learn"

I creates two documents using MS WORD on my Mac.
The extension it put on, apparently automatically,
is .docx

Works fine and they both open fine.

Now, using Bootcamp, I am in W10.
Have these documents, also, on a USB stick,
Tried opening them from the USB.

Wordpad won't open them.
LibreOffice Writer will.

The extensions seem to have changed.
Now they are: xml and XLSX for a second document I tried.

What are these two extensions ?
Why did it change going from Mac OS to W10 ?

Shouldn't MS WordPad work ?

Thanks,
Bob