I wonder how the XenForo developers made this forum software

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I am trying to make my own forum software, but I noticed that this is really hard. And, I wonder, how the XenForo developers started making forum software? What is your advice for making forum software? I can not find any documentation or thread about this topic, I am asking this question in here because I think asking question to the XenForo developers is better way. Also this is my first thread, I hope I am posting this thread in the correct forum.
I wonder how the XenForo developers made this forum software

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