Looks like sRGB scaling isn't being done right
If you do this:
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Take a 4K video with lots of colors at different brightnesses.
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Upload it to YouTube and wait for processing to complete.
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Play it on YouTube in Chrome at FHD (not 4K). Stop at some point, say 10 seconds in.
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Play it on VLC at FHD. Stop at the same point.
Then you will likely notice that YouTube has better color saturation. I suspect this is because you're downscaling 4K to FHD using pixel averaging, when in fact the sRGB specification doesn't allow this. That's why you end up with slightly washed out colors. So for example, flesh tones become more uniform, while YouTube will show more variances.
Proper sRGB scaling is complicated and requires meticulous adherence to the specification and proper attention to floating point rounding and error dithering. The specification provides precise conversions between sRGB and intensity, so it's possible, just not straightforward. The conversion is indeed approximately linear, which is why so much software does it wrong but looks OK upon cursory inspection.