They work on the same points system as vehicles and the emblem, but drop regardless of win streaks and can be obtained from playing on the trials passage rather than lighthouse. You can get a once weekly 50 points for visiting the lighthouse with a 1-4 win streak, and another 100 points for a 5+ streak. Then it’s also a farmable 1 point per win goes towards a ghost shell drop. It’s hard to say the exact drop rates without Bungie releasing their equation as the points system is non-linear (e.g. 300/1000 points equates to a much lower drop chance on a win than simply 30%). It’s 1000 points total for a guaranteed drop, but you’ll likely get one by around 300-400 points.