Am I the only one getting lost in all the customization options offered by smartthings? Specifically the rabbit hole that is smartapps and device handlers. I can’t make sense of most of it. I feel like I’ll find something useful and yet, I’m not sure I do. I guess smart stuff offer too many automation options. My head is spinning!
Smartapps were important for the first couple of years of smartthings, but you probably don’t need them now. Most people don’t use any except the ones that are part of the official features. 
Try doing what you need just with the + for automations in the upper right corner of the main screen in the smartthings app. Then if there’s something that you want to do that you can’t figure out how to do there, come back to the forum and ask and somebody will help you.
There are a number of other ways to create rules for SmartThings, including WebCore, the Rules API, and custom code, but again, most people won’t need those.