Even lemax2 which is 2 yes oldies playing 4k videos in YouTube. But Nokia 7 plus doesn't play, it shows only 1080p as the maximum permissible quality even in pure 4k videos of YouTube. Nokia kindly sort this problem with an update play..
In any mobile phone youtube app plays at maximum resolution which is device resolution. For example 2160p will be maximum on YouTube for 4k display, and 720 will be last for HD display. Mobile phone has nothing to do in this. And for recording 4k you need a 4k capable camera sensor not a 4k display. Many action cameras record 4k, do they all have 4k displays?
And for second point, why you want to play a 4k video file on a 1080 p display, because that will be downscaled to 1080 p to play on that display. Their will no difference between playing a 4k or FHD file on FHD display.