Also anything you generally find relaxing that isn’t intended to be.
For reasons I cannot articulate, I never liked intentional ASMR videos, but give me an upload of a three hour video course about a word processor from the 80s or a training video about how to take a sonogram of the kidneys and I’m all in.
As for generally relaxing stuff, for some reason I find recordings of radio comms, like storm spotter nets, to be relaxing. Also local news coverage about major weather events.
And to you UK lemmings I know about the shipping forecast.
ProWalk Tours. Just walking around in a foreign country in Ultra HD. No commentary. For me the sounds of walking and the mumble of genuine chatter is relaxing. Plus if you look at the screen every frame is beautiful.