You ask for something very complex and subjective. For instance, I have a set of libraries written in Python, and I use them in a web app, on a desktop app and

You ask for something very complex and subjective. For instance, I have a set of libraries written in Python, and I use them in a web app, on a desktop app and in Android (using python 3.5 provided by crystaxNDK - https://www.crystax.net/ https://www.crystax.net/). The python libraries are my "backend" where I have most of the interesting code, and only the frontends change: html/css/js over flask for web, PyQt for desktop and Java/JNI for Android.