Why do videos look extremely pixelated on small screens?

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on the music i get off of Utube i have to do some 'puter work with it to make it sound passable.

Sometimes I'll double-up a Youtube track in Audacity to give it an extra thickness. I'll make the double track a bit lower in volume so the final output isn't too distorted with clipping. Look into adding some dither with your Youtube downloads.

what works for me is, if it is a monophonic track, stripping away the difference signal [anything that is not strictly center channel], the difference signal contains the bulk of the noise and distortion. that by itself can make the audio quality markedly better. on stereo tracks, i do a relative of that, getting the idea from how FM radios have a treble blend circuit to reduce noise on weak stereo signals, i progressively blend the trebles starting at about 4K, with peak reduction at about 8K, removing the noise-laden difference signal in that range, then take the portion that i blended-into-clean mono, and stereoize it via comb filter and other techniques, then mix that part back in to give a semblance of treble-range stereo sound to id. sometimes i also have to add a subtle chorus to that range to make it sound less gritty. since the ear tends to be less sensitive to distortions above about 8k, i get away with it.

Oh okay, is it possible to do this pixel mapping to the videos before I send them then?

yes, that is the only way to do it. now as to HOW, well that is way above my pay grade. but it all boils down to optimized bit-mapping.

Hi. I work in video production.

it's possible that the downscaling itself is creating the problem, but actually, it shouldn't - the video shouldn't be visually pixelated, but blurry. (since most downscaling will average the higher number of pixels that make up the area that will eventually become a single pixel in the downscaled image).

Much, much more likely is that you're looking at very poor compression. Whatever you're using to send the video decides that the filesize is too large and converts it to a smaller format- smaller in resolution, but also data rate and quality. Since phones are low power and video compression can bring my heavy duty work computer down to its knees, don't expect your phone to be able to handle 4k video at all, really. I know new phones can record 4K, but that's it, really- process on a computer after that.

troubleshooting:
1. try not sending a 4k video, but an HD or even smaller video and see if the problem persists (your app might find an HD video small enough to send and not perform conversion)

2. send a shorter (like, a second or two) video and see if the problem persists ( the app might merely detect a filesize that is out of bounds and not care about video resolution)

3. find out how large the file you're sending is in Mb (anything above 50 Mb is really not fit for sending through messaging apps)

4. make a screenshot of the pixelated video and post it here, so we can have a look at what kind of "pixelation" we're looking at.

Whilst the first part of that statement is true, it doesn't apply to most real world scenarios. Most displays have nowhere near the maximum density of pixels possible for their display area. Instead, they have the pixels necessary to meet a certain specification. There's no shortage of small displays out there with resolution parity with their larger brethren. In fact, the displays with the highest pixel density available to consumers tend to the smallest. Thus I would say the second part of your statement isn't true.