Upscaling creates enough output pixels to fill a larger display. It can improve edge presentation and reduce obvious pixel structure, but it cannot recover original detail that was never recorded or was removed by compression. Native detail and an enlarged output are different things.
Understand the enlargement step
A fixed-pixel television must map incoming video to its panel. When a 1080 p picture fills a 4K screen, the scaler estimates additional samples between the originals. Different televisions and streaming boxes make different choices about edges and texture, which explains why the same stream can look slightly different on two devices.
Choose a sensible starting point
Use a normal picture preset and reduce extreme sharpness before comparing scaling options. Strong enhancement can make a picture appear more detailed at first, then reveal white outlines around text and faces. Watch a slow camera movement as well as a still frame; unstable texture is easier to notice in motion.
Compare one scaler at a time
If your box offers a native-resolution or automatic output mode, compare it with a fixed 4K mode using the same material. The television may scale one version and the box the other. Keep the version that looks cleaner without introducing delays or unreliable switching between programmes.