Some are quest based. One is elite bounty based. Most are acquired by using a three of coins before killing a boss like at the end of a strike, prison of elders boss or raid boss. Open world bosses work as well. To get three of coins, you must have strange coins to buy them. 7 strange will buy you 5 three of coins. That is 5 chances for an exotic to drop. Xur sells three of coins and shows up every weekend at a different location. To get strange coins, more often, play prison of elders, do the mystery patrol on each planet and open resource chests around patrol zones. Level up your factions for four strange coins each rank up. The most strange coins I’ve ever gotten out a chest was 6. The most I’ve ever gotten from farming strange coins for a week was over 450. The most exotics to drop in one day for me was 8 when running strikes while using 3 of coins. Hope this helps.