ELI5- why do screens/lights flicker in videos taken by phone or even yt vids, but not in movies and professional productions?
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This is to answer the first part of your question. Screens flicker in video taken by phone or any other video capturing device for one reason only. The frame rate of the video recording does not match the hertz rate of the location that is being shot. I battle this daily with the video team I work with as we shoot in different countries regularly. Most of the younger generation might not know this as most videos these days is consumed online which will play any frame rate and anything progressive or interlaced all the same, but back in the old days, each country had a standard when broadcasting video.
There are several broadcast systems across the world but 2 of the main ones are systems called PAL and NTSC. These are systems based on frame rates that are derived from the cycles of the electricity, aka hertz. For example, in the US, which generally has the supply voltage 120v has a cycle/hertz of 60. This means all lights, screens and anything else that has cycles will flicker at that rate. NTSC is generally considered to be 30fps or 29.97fps if you use drop frame. What is 60 divided by 2? 30. This is the reason NTSC is at the frame rate which means if you are shooting native NTSC, or 29.97 (or 24.97fps for that matter) you could shoot anywhere in the US and there is a good bet if you keep your shutter speed native to the defaults you will not suffer any flickering.
PAL on the other hand (think Europe or some places in Asia) works in locations that have 240v supply voltage. Also 240v in these locations are at 50hz. What is 50 divided by 2? Yes, 25. Guess what the frame rate is of PAL? 25fps. This ensures if you are shooting in a PAL aka 240v supply location, you will also not suffer flickering in most cases. Oddly enough, there is a reason for these systems. Of course this is not a constant, you will have the odd flicker of halogens, also fluorescent tubing and in I most cases, changing the shutter speed slightly will fix this but might not always be the best choice when it comes to low light situations.
General rule of thumb is, shoot the frame rate of whatever the hertz/supply voltage is in the country you are in and you will probably be ok.
Phones tend to shoot in a non fixed variable frame rates. Not all but a majority do so you will find flickering can happen in some recordings but not in others.
But for films, well films have budget, budget pays for very talented people to find ways to sync refresh rates of screens to match the frame rate of the cameras and the everything is perfect.
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