Bootcamp choose Windows ISO opens ISO instead of selecting ISO
You appear to have created USB flash drive Windows 10 installers by using both rufus and and the Microsoft media creation tool. Both of these methods bypass the use of the Boot Camp Assistant. You are suppose to use the Boot Camp Assistant to copy the files to the USB flash drive.

You should go to this website from Microsoft and download Window 10. The last time I checked, the October 2018 update (1809) will not work with BootCamp. This only applies to older Macs that use a flash drive to install Windows. This includes your Mac. So you will probably want to download the April 2018 update (1803).

If you have High Sierra or Mojave installed on your Mac, then you can also create the USB flash drive installer without using the Boot Camp Assistant. The instructions are given here.