Here’s the community FAQ on adding custom handler’s for the original smartthings platform, the one that uses groovy. That’s the copy and paste method.
The examples shown will be from the older app version, but the concepts in the process described will still apply.
[OBSOLETE] FAQ: An Overview of Using Custom Code in SmartThings (SmartThings Classic, Groovy Code)
Smartthings is right in the middle of transitioning to a new architecture. That will be Edge. For that one, yes, it’s just download. You don’t need to do anything more than that. But it’s still in beta, so some of the edge drivers will be glitchy right now.
The transition will probably be complete in a few months but no exact timeline has been given.