What's the best way to approach making a loot box program using C#?

I'm learning C#, and something that I really want to learn how to make is a loot box program. The best way I can think of to make this is making an enum and assigning numbers to the items in the enum, and then randomly generating a number. Then if that number is within the scope of one of the items in that enum, you receive the respective item.

Something else that I would love to learn how to make is something like a Pokémon game. If something happens, like running around in the grass, and then you encounter something, how does the program select which Pokémon to give you? Is it similar to what I described above?