1) It's pretty much what you'd imagine, though it is a bit more... chunky. Imagine the star fragments from Animal Crossing or Breath of the Wild, something like that, but easy to collect a few in your hand.

2) They only appear around this time of year, which is why they're associated with Tanabata in Jigokucho.

3) We'll say about a handful is all you need to start Feeling It, and more than that it will become harder to just ignore and move on.