The Monte Carlo debuted as a PlayStation-exclusive Exotic weapon in the original Destiny fashion in 2022, but has now returned in full glory to Destiny 2 with the release of the Shadowkeep expansion. Unfortunately, there is no predetermined way to get the Monte Carlo as it is only available as a random deposit. There is no associated Exotic quest and there is no Pinnacle quest to grind for this weapon. It's just good old RNG for this one.
How to get Monte Carlo
As we previously stated, the Monte Carlo can only be obtained in Destiny 2 as a random Exotic drop. Exotic gear is the rarest of the rares in Destiny, so you'll have to be really lucky to get the Monte Carlo. Thankfully, Season of Dawn has made some much-appreciated adjustments to Exotic Engrams that will make weaponizing a lot easier for long-time Destiny 2 players. Now each Exotic Engram you obtain will act as an Isochrone Engram, meaning 'he'll give you an Exotic you don't already have. If you already have a vault stuffed full of Destiny 2 Exotics, you're much more likely to get the Monte Carlo from an engram than someone just starting out.
Luckily, you don't have to just sit around and hope an Exotic Engram lands on you. There are ways to secure an Exotic, even if it takes a lot of time and effort. Xur is going to be the easiest method. Every Friday, find him and search through all 97 Legendary Shards for an Exotic Engram. The Exotic you get is still random, but at least you can roll the dice every week that way. Your other best bet will be to upgrade your Season Pass. The Season Pass gives Exotic Engrams at certain ranks, and while it can take a long time to level up, it beats hoping you're lucky enough to get one after an activity.
Hope you don't have to wait too long to get your own Monte Carlo. If you're unlucky in getting Exotic Engrams, then work on your Season Pass and visit Xur every weekend. It may not happen on your first try, but it will definitely happen one day.