I want to join this amazing platform. Can I get free codes for making an app or I have to pay to the owner of the code.
What do you mean by “free codes”?
You can sign up on GitLab and start using it for free.
Everyone develops its own application, you cannot just “buy” someone else’s code. GitLab is not a platform for making an app by buying modules and putting it together by clicking. For that exist some other platforms.