Welcome to the forum @karagozkenan! In Defold you also publish to multiple platforms from a single code base, and when it comes to in-app purchases as in your example you would have a single code path with a call to iap.buy() to buy from Apple/Google/Amazon and Facebook. There is no need to handle purchases differently between the platforms. The engine will take care of the details for you.
BUT, with that said, you can call sys.get_sys_info() to get a Lua table back with system specifics, such as OS, model, language and so on.
Defold doesn't have support for dynamic content and automatically picking the correct version of an image based on screen resolution. This is discussed in multiple forum threads, and some solutions are suggested, for instance this one. We will implement a system similar to what you have in Corona, but I'm not sure when.