Map purchasing very confusing

I am considering purchasing a Garmin device. I thought trying this app app might give me a chance to preview some of the features on my Mac before I spend the money. It seems like the software could be good. However, I cant figure out how to purchase any maps without already owning the device. It says you can and download them, but when I get to the website, it wants the code for the device. Does each device use different maps or do they work on all Garmin products? I wish this was clearer.

skellener about Garmin BaseCamp