This is built around guac but more of a managed version with a custom frontend and browser client.
if you install Guacamole on a server, what kind of software to be installed on machines that need remote access?
there's no mention of how to access windows machines, even thought it's in the demo?
There's a HTML5 browser interface. With some basic scripting/"programming" skills and some front-end magic anyone could build such a service. More important is a good network and security engineer to keep it safe and stable for everyone (around the globe).