This page of the trophy guide for Far Cry 6 explains how to unlock the Finders Keepers! trophy. It involves stealing and bringing back 3 enemy FND vehicles with resources in good condition. This page contains detailed instructions on how to find and steal the vehicles with resources.
This trophy is for stealing selected types of trucks. These can be machines with a "shiny" trailer (example in picture 1) or FND Convoy, whose icons can appear on the world map.
Look around large paved roads, such as east of Montero Farms in the Madrugada region.
If you've chosen a convoy, it will consist of at least two vehicles. However, you only need to get the resource-transporting truck.
Before you start attacking any truck, pick a relatively fast vehicle from the garage or steal it.
Don't fire at enemy vehicles when you approach the convoy. Instead, catch up with the truck from the passenger door. There should be an option to Hijack a truck, activated by pressing the right analog stick.
Your character should automatically jump into the cockpit and eject the enemy/enemies from it.
You now have 2 options - you can start running away immediately, or you can get out of the truck and attack the enemies from the jeep that accompanied it.
Delivering stolen trucks
With the truck, you need to get to one of the Vehicle Pickup locations. You can discover their icons as you progress through the game, and they may be located e.g. next to one of the guerrilla bases. When you reach such a location, park the resource truck in the large rectangle - it will automatically be unloaded and you will receive the crafting resources that it carried.
Repeat the described steps with any 2 other resource trucks to unlock the trophy.
Additional note - This trophy requires the trucks you deliver to be in great condition. However, you don't have to be careful at all times while driving. Shortly before you reach the pickup point, you can get out of the truck for a moment and use a repair tool on the vehicle to get it back to 100% working order.