You charge the Beams with a USB-C cable. A cable is included. To charge multiple Beams at the same time, it is useful to use an adapter with multiple USB ports.
No, that is not necessary. In the app, you can use Standard, Battle, and Tap & Track modes. The Standard and Battle modes work without straps; no data is stored. The Creatives (making games yourself) also works without straps. However, if you want to store data, link it to personal player profiles or use the leaderboard, Skill Tags are necessary.
Yes, the Beams are splash-proof. So they can simply remain standing during rain or under sprinklers.