How do I install Windows 10 on a newly assembled PC?

How can I use a Windows 10 key? Doesn't that work without USB or does it work without it. And if so, how

I don't have any Windows 7 or 8.1 portable laptops or PCs

I would create a bootable USB stick and then boot the PC in the BIOS using this USB stick.

You can use Microsoft's Media Creation tool to create a bootable USB stick. Alternatively, as far as I know, you can use it to create a DVD

Bootable? Do you mean that Windows is already on the USB stick or what?

You can simply download the Media Creation tool from Microsoft and select the USB stick there. The software correctly configures the USB stick and saves Windows on it