So basically what is happening now. The JS allows it to happen on the client side, taking a HUGE load off the server trying to parse the ua string, find the right translation, bring up the file, and respond. They probably have a single binary that has the HTML, CSS, and JS all compiled in and update it once every year.

I definitely agree here. But if you want to better learn how the system works, I suggest getting a cheap used laptop and just hammer at installing arch using the install guide, not the installer tool. The tool is nice and all, but you still need to know what is going on under the hood and to understand most of it.