re: Chromecast or Firestick?

No, if you do get a letter from your ISP, you do not need to respond or otherwise contact anyone (unless the letter tells you to), but you should take action to prevent getting another letter (e.g., VPN). Some ISPs have a 3-strike rule before they terminate your account. Others, like Cox, have a less-definitive policy (I've heard it was a 10+-strike rule with varying consequences). The letter will tell you what you were caught pirating/sharing, so you'll know where you went wrong. I used to torrent on public trackers pretty freely, but out of the blue I got an infringement email from Cox twice in one week years ago. So, I started using a VPN and/or private trackers. If I ever get a letter for something I streamed from a Kodi add-on, I'll start using a VPN for that, too.