Android : Using Game Console Emulators in Android Application

I've asked this question before but it was seen as a broad question. So I'm asking a more specific question about that topic now.

I want to build an Android app with ability to play Sega Genesis ROMs. Obviously it needs a Sega Genesis Emulator. Based on what I've seen in other android apps with Game Console Emulators, I've figured there's no need to write an emulator from scratch. For example this app has emulators for more than 10 Game Consoles and obviously they haven't spend years to write +10 emulators from scratch and put them in an app just to distribute it for free.

So I'm guessing there's a way to use (include) an already written Game Console Emulator in an Android application. Is that right? And if it is, how one can do that?

I've search A LOT and found nothing about it. Maybe this process has a special name that I don't know and I should search with that name.