Video games are very "noisy" in terms of image output, tons and tons of approximations and high precision but limited "accurate" solutions are used to render out the result and as such tons of factors to consider, too many to really give you an "exact" one thing but a very very simple one is a natural anti-aliasing as a result of downsampling.

Much of this has to do with the fact that we are rendering out to "pixels" and geometry doesn't always fit perfectly and as such filtering is needed to clean up the artifacts.

If your game is running at say Full-HD (1920x1080) and you have absolutely nothing else being done to the image (no anti-aliasing, no mip-mapping, no anisotropic-filtering, etc.) you'll have a bit of a pixel soup of a scene (some textures will be blurry, others will be pixelated, and you'll have "jaggies" across most non-transparent objects in the game).

You can easily solve a lot of these problems by simply using a buffer that is 4x larger than your target output resolution, drawing your game scene to this and then sampling it down to the target resolution.

This is usually what we call "super-sampling" and there are several ways to do this when you downsample down you merge or exclude information downwards and much of the sub-pixel information from the source information is transformed.

If you have a camera at your home you can effectively do this yourself in the real-world; take a photo, open said photo in say photoshop, resize photo down 33% and you'll notice image quality on the resized photo is usually improved (this is also for a variety of reasons, mostly ISO noise though is reduced heavily).

It's just not widely utilized because it's computationally expensive and it isn't always the case that all of a games shaders scale with the increased resolution (or in rarer cases, certain targets can't be created because some shaders could already be overdrawing).

If you want just overall a... "glimpse" into how complicated it is to render a game scene I highly recommend this post by Adrian Courrèges which will give you a deep but IMHO high level explanation from a semi-modern game: https://www.adriancourreges.com/blog/2015/11/02/gta-v-graphics-study/