Undetectable payload

[bingopl]

Hi, I wanted to learn how to perform ATP attacks. I have a problem with creating a backdoor / payload that will not be detected by antivirus systems and which will give me access to the attacked computer via a reverse connection. 

I'm not a good programmer, but I thought I would create my own code that would give me a system shell using the SSH reverse tunnel. I find that if it will be my code it will not be detectable for AV.
Unfortunately, the more I think about it, I come to the conclusion that once I have such a system shell, I still have to provide a meterpreter payload, which will allow for more advanced work, and it will definitely detect the anti-virus. 

I want to learn but I do not know what to do in such a situation.  How to create a meterpreter payload that will not be detected by AV?

I see that msvenom generates already encoded payload.  Is it possible to get such a payload not encrypted? Maybe if I changed his sources in pure C, it would be less detectable? 

 

I've tried veil, shellter, thefatrat, etc. But it does not help. Most AV immediately detect payload. 

I am mainly interested in payloads on MS Windows. Later I want to try Android.