Not scientific, but when I snoop, I most often see the thumb scrolling and various incarnations of the video shorts apps like TikTok, YouTube Shorts, Instagram, and Snapchat.
Probably any of the above except work? It's indeed a tool for any kind of distraction you want/whichever road you take (i.e. whichever app you open) you'll find said distraction. HN counts too. (Ironically I'm typing this as I sit on the toilet.).
I agree on HN, though I think it's markedly different as it moves slow enough that you can just check it once or twice a day and get most everything you want. Whereas a lot of other social media is an unlimited feed of everything, ephemerally, and can make you feel like that if you blink, you'll have missed something.
I do check HN from my phone, typically in the mornings and night when I have downtime and don't want to watch TV or read a book. But I can't say I've ever been at a park, bar, or airport lounge and just itching to see what was going on HN this second.
My wife's vice seems to FB, but unsure if that's what everyone else is doing.