Alright I watched 75% of the video and I really made an effort to follow along but I really think there is a lot of nonsense in it. Maybe some parts are legit but since he also says a lot of nonsense you can't really tell the difference between what's real and what's false. He throws some statements out there with such conviction making it seem like real scientific facts supported by real scientific evidence but he probably just made them up. For example, he says Blue light makes vitamin A, Vitamin A is yellow, Blue light makes you fat.. Especially the last point makes no sense at all. Saying that UV rays affect people's weight is just ridiculous. I personally avoid the sun like the plague and I weigh less than 50 kg. I found these articles that you could read a little bit about the subject, at least there is all the bibliography:

PubMed

The Melatonin-Mitochondrial Axis: Engaging the Repercussions of Ultraviolet Radiation Photoaging on the Skin's Circadian Rhythm - PubMed

Sunlight is a vital element in modulating the central circadian rhythm, such as the regulation of the host's sleep-awake state. Sunlight is also co...

https://www.mdpi.com/2076-3921/10/6/920

How UV Light Touches the Brain and Endocrine System Through Skin, and Why - PMC

The skin, a self-regulating protective barrier organ, is empowered with sensory and computing capabilities to counteract the environmental stressor...

This guy is definitely overstating the role of UV rays and completely ignoring the damage the sun does to your skin. So, going back to the story of training your skin to the sun, it's the same thing, it's not supported by scientific evidence. Maybe your skin tolerates the sun better or you feel like you get fewer sunburns, but the damage at the cellular level is there. you're really don't want to wake up one day with your skin completely damaged and you'll feel like you look 15 years older than people your age and maybe you'll think, yeah maybe it wasn't a good idea to listen to a charlatan.