https://www.microsoft.com/en-us/software-download/windows10ISO

So if you're trying to download windows 10 ISO files from Windows 10 / an updated browser, microsoft will send you to their landing page where they want you to download their tool. But you want the ISO.

To get the ISO, install an agent switcher and switch to an old Internet Explorer User Agent. Visit the Windows ISO page again. Now you can fetch the ISO.

Use Rufus to make the boot device.