What to learn for my Project

I’m a Software Engineer and I have a project in my mind that I’d like to build. The project will require the use of an external device that will interact with a mobile application. Now, I have done some mobile development in the past so I can navigate my way through that. My question however is, how do I build a device and make it interface with my app. Systems Programming? Any help will be appreciated. Thanks.

Side note: If anyone is looking for a side project to work, please hmu. Thanks