This parameter specifies which display to scale, so only the built-in display will be scaled. Running xrandr without any parameters returns all available outputs, as well as the resolutions the currently connected displays support.
I don't know about that; I use just one screen (laptop or HDMI, not both at the same time which is presumably what you're referring to) and it works for that. That's not really what the previous person was talking about either.
If you have two monitors with very different DPI, for example, I almost poke my eyes out when I tried 5k and 1440p together, you only have two choices: render for 5k and scale down to 1440p or render at 1440p and upscale to 5k. Well, you can also pick a middle ground that makes both monitors look blurry. Either way, at least one monitor will be _very_ blurry.
At the office I plug in a monitor over USB-C and that just works on my X11 laptop. If something in a browser on the monitor was too large or too small I'd just zoom in/out until it was fine.
