Dropsy has to do with osmotic pressure, the fish loses it's ability to not absorb excess water, it can be a bad thing and unfortunately can be common in bettas, stress can bring it on like being moved into a new tank and if water conditions are too different in moving them, or some just have something internally going on.
I will usually use aquarium salts to help take off the pressure, I have read about others using Epsom salts but haven't tried it, I will add the recommended amount of salts on the package, usually 1 teaspoon per 10 gallon, than add more each day for several days than do a water change to lower it. It doesn't always work and some don't recover.
Dropsy can also be brought on by some fungal as well as bacterial issues that become apparent during stress, there are tablets that can be used to treat these infections and you should probably keep some on hand. Since you have snails I would try raising the temperature of the tank a bit to see, some bettas are so stressed by sitting in the pet shop that they don't all make it.
Bettas are short lived fish, about two years old, but most live about a year or so depending on the individual as when you buy them they are already mostly mature.
You will have a sharp learning curve with keeping fish, read all you can and try to understand all you read, once you get it, it all will be easy, or at least you will know what to do.
