In order: Bilinear, nearest-neighbor, waifu2x, nearest-neighbor on pixel art.
To artificially increase and modify aspects of uploaded content. By definition, content under this tag is discouraged to be uploaded on to the site. As upscales are generated from the original file using computer alghorithms, it means that it can be done again from original file, but original file is impossible to obtain from upscale.
If you have upscales, please keep them under your personal usage scenarios. Only if the upscale is only version exsisting of the post or artists own doing, it can be accepted but will still be highly discouraged.
Most common form of upscaling. There are several alghorithms which can be used to make image larger, most common general purpose ones are bilinear and lanczos and most popular with drawn content waifu2x.
To tell upscale from original, usually it's good idea to inspect both files. If general upscalers are used, whole image usually looks really blurry. If more advanced upscalers are used, then parts which do not have enough data look blurry, e.g. really thin line endings.
Only lossless upscale with resolution is nearest-neighbor scaling (also known as "no filter" upscaling) with whole multipliers. On the site, this is only acceptable method with pixel_(artwork), where 1:1 pixel ratio will look tiny on modern monitor resolutions, with other artwork this method will cause unwanted aliasing. All other upscales are subject for deletion if there isn't good reason for said upscale.
Frame blending is usually common to happen if frame rates do not match when creating video and end frame rate is larger than original. Usually with this you can pause the video and have 50% change of seeing two frames on top of each other.
Another form is frame/motion interpolation, but has several other names. This means taking two exsisting frames from the video and then guessing what could happen in between them. This means that instead of seeing two frames on top of each other, there's usually slightly blurrier frames in between exsisting ones and sometimes if the prediction goes wrong (e.g. sudden high amount of movement), you can see morphed frames of two frames blend. (example: post #917295 from 0:23 onwards).
Modern TVs usually have this technology implemented in with regular presets outside gaming mode, so usually with the models all you need to do is watch the video regularly. With PC, it's possible to use software which does this on fly: