The script will use msfvenom (metasploit) to generate shellcode in diferent formats ( c | python | ruby | dll | msi | hta-psh ), injects the shellcode generated into one funtion (example: python) "the python funtion will execute the shellcode in ram" and uses compilers like: gcc (gnu cross compiler) or mingw32 or pyinstaller to build the executable file, also starts a multi-handler to recibe the remote connection (reverse shell or meterpreter session).
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'shellcode generator' tool reproduces some of the technics used by Veil-Evasion framework, unicorn.py, powersploit, etc,etc,etc.."P.S. some payloads are undetectable by AV soluctions yes!!!" one of the reazons for that its the use of a funtion to execute the 2º stage of shell/meterpreter directly into targets ram.