The user agent is some information the browser sends to the website, which has information about the operating system, browser version, etc...
Microsofts website won't give you direct access to Windows 10 ISOs if it detects from the user agent that you are using a Windows PC, it will instead direct you to the media creation tool (which athough that can generate ISOs, these aren't the same as the ones you can download directly if you use a non Windows operating system
I would suggest using a browser extension, as it makes it easier, all you need to do is change it so that it reports that you are using a Mac (or Linux) computer, then it will give you access to the direct ISO downloads
When using Microsofts website, you will only be able to download the latest version of the ISO, and with the Rufus tool i mentioned (which is a tool for creating USB drives, but it also has an ISO download feature), older versions won't be able to be downloaded through it, as Microsoft have apparently either removed the older ISOs from their servers completly, or blocked access to them in some way