ELI5: How do PC emulators run games at a higher resolution than they were originally made in?

Microsoft announced Backwards Compatibility earlier this year, boasting proudly that games would run on Xbox One "exactly as they did on 360" via what they referred to as a "high level emulation" and now PS4 is apparently going to be emulating PS2 games in a slightly "upscaled" resolution.

This makes perfect sense to me. You simulate the environment of the orignal platform and run the game as it existed then.

And while I don't quite understand "upscaling" in technical terms, I understand that you can artifically raise the resolution, though it won't look as good.

This would be comparably to when a DVD is played by a Blu-ray Player, would it not? BD players often have the capability to upscale DVD movies. At least that's the term I've often heard people use. Sometimes movies are also released on blu-ray upscaled, which people then complain about as it is no "true hd remaster or restauration".

And I understand that without considerable work, i.e. an "HD remake", this original product exists in the resolution it was made in and there is only so much you can do. Hence the Xbox One runs 360 games that ran in 720p still in 720p and PS4 emulates those PS2 games in an upscaled 1292x896 from their native 512x 48 and 640x448.

How then is it that an emulator on PC can run my Twilight Princess, which is a 480p Wii game, in the most stunning 1080p? Is that also an "upscale"?

Submitted by fynsquery Xbox 720 Release Date 2015