If you need most simple, think of the end result of a game being lets say 30 frames per second (images). Upscaling those 2D images to a higher resolution, or to strech the images into any direction is something my smart toothbrush can do.
(It gets harder if you want to apply good shaders with 6 passes in real time - but thats something only retroarch nerds do, and no one else.
)
To get better polygon models, better textures, less blurry fonts, or a main Character model in FF7, where you can make out the eyes..
You need a higher rendering resolution. In 3D games, that is - for 2D games, its "the same". This also makes bilinear filtering perform "better", because it has more detail to work with.
Currently (for the Switch) its in Mupen Plus Next (N64), PPSSPP (PSP), but not in PCSXRearmed (PSX). Its not in PCSXRearmed, because the core got viable (performant enough) on the Switch before Jit was a thing. Jit benefits from GPU acceleration, which the current PSX build does not.
And thats what you need because running a game at a higher internal resolution is more performance heavy.
Patches - are patches.
(Nothing special about patches.
)
edit: Also there is the thing that "higher resolution is the best anti aliasing" (what nvidia brands DSR), because you basically could render a game in 4K, then downres the output image frames, and have better AA, than to make up in between pixels, that werent really calculated. This then also plays into contrast and depth perception - which means, if you do this with games, they can really look more pleasing, even if you do nothing else them than run them in a resolution higher than anyone had expected them to be run at. And then downres them.
Push internal resolution too high and edge definition might get too harsh for some games, Zelda OOT in 1080p (internal resolution) above is one such example, people might prefer a tad under 720p, which gives you a "softer" look:
(Compare to the Zelda video in 1080p, also posted on this page.)
