At what point does a “customized version” of OpenSees become distinct from the main OpenSees? After tinkering with OpenSees, when does it become “your own”?
In most cases, the answer to both of these questions is “never”.
Sure, you will learn a lot by implementing Concrete24, even if it is only an incremental improvement over Concrete23. You’ll also learn a lot by integrating OpenSees into your daily workflow.
But, you didn’t build that. And neither did I.
Obama’s “you didn’t build that” was taken out of context and used against him in the 2012 US presidential campaign. Of course he was referring to the roads and infrastructure that support commerce, not criticizing entrepreneurial success.
Unless it’s blatant plagiarism, I’m not going to criticize someone’s success with OpenSees.
Just remember all the infrastructure–integrators, algorithms, elements, interpreters, support, examples, bug fixes, etc.–that you never had to think about while customizing OpenSees into your own.