Apple TV 4K: Dolby Vision 4K (50Hz) OR Dolby Vision 4K (60Hz) OR HDR?

Why is 50 Hz selected by default? And what is the effect of manually selecting 60Hz?

50 Hz is probably default because of the country or region and the historical TV standards there (PAL/SECAM), as a result of the historical frequency of the electrical system. If you have a TV that supports 120 Hz, then you could try 60 Hz, as that divides neatly.

There are also 4K HDR (50 and 60Hz) options. But if I understand everything correctly, then Dolby Vision is better than HDR.

The HDR setting is for the HDR10 standard of dynamic range.

The arguably more promising Dolby Vision standard of HDR is offered when that capability is detected (in the TV). HDMI cables or devices in between may influence the detected capabilities.

You will only get options that are compatible with your setup.

There is one TV channel on which, as it seems to me, the image does not have smoothness, but slightly twitches.

You wouldn’t know what they did with the video signal before it was streamed to you. Maybe the source is in error, and it may not be something fixable on your end.

It also seems to me that when watching the series on the Apple TV+ service, the sound lags a little, maybe it seemed.

That shouldn’t happen. It should be synced for all content providers equally. If post-processing would cause a delay, then it would be the same for video as for audio, in order to keep sync.

Did you watch with original audio, or a different language audio? Do you perceive the same effect with other audio tracks?

Tell me, maybe it is still worth making some additional settings that would not harm Apple TV and TV).

I would advise to try the Match Content settings: Match Dynamic Range and Match Frame Rate. Match Dynamic Range would send SDR as SDR to the TV, HDR10 as HDR10, and DV as DV. Same as source, without conversions. Match Frame Rate would send the source frame rate to the TV, without conversions. If it gives unwanted effects, then you can change the setting accordingly.

Is this frequent switching harmful to TV or Apple TV?

Not at all. Older TVs may have trouble with displaying some frame rates as intended; if that happens, then don’t use this feature. Most content will be 24 fps/25 fps/30 fps, and your TV may ‘upscale’ that to 100/120 Hz anyway, so it doesn’t matter much if the Apple TV ‘pre-scales’ that to 50 or 60 Hz.