Nobody in Null Byte Haven't Binded 2 Apks?

How can we bind our metasploit android payload with another legit apk? About 10 nullbyters are asking this for weeks, maybe months. We couldn't find straightforward guide for that. We somehow need to run our android reverse tcp payload as background service. But we don't know how to do it.

Please don't recommend AndroRAT. We'd like to understand the process.

I study SPF to understand process. However, it doesn't actually bind two apk. It just backdoors a legit apk with pre-prepared services. I need to learn how can we convert our ACTIVITY to a service. Yes, there are some guides to do that. But I'm looking for a spesific guide which someone creates services and binding them with a legit apk. Otherwise, with my beginner coding skills I'm just staring at the guides and don't understand anything. If you did this before, can you send me manifest and smali of your binded apk? So that I can understand the process.

I'm currently learning C and to avoid mixing things up I don't want to deal with learning java as a beginner. Please help me to do this.