I have seen this many times that the daytime running LED headlamps or taillamps of cars blink in slow motion videos, why is that?
It's how they make the lights brighter or dimmer depending on whether they are acting as daytime lights (dimmer) or as headlamps at night (brighter). They quickly turn them on and off fast enough that we can't see the flickering, but they change how long the lights are on each time they flash so the apparent brightness goes up or down. If you want your lights to be 50% dimmer, only turn them on half the time. They are actually flashing very fast, likely >1000 times per second.
This technique is called pulse-width modulation (PWM): https://en.wikipedia.org/wiki/Pulse-width_modulation