Did all the founding fathers write the United States Constitution or one individual?

No one person. The constitutional convention included delegates from twelve states and they spent weeks hashing out the various political issues and deciding how the government should be structured. Once they finished with that it was turned over to a committee of style to put it into nice neat legal language and orderly language. Gouverneur Morris provided much of the final wording even though most of his ideas did not get into it. He had wanted the president to serve for life and have the power to appoint senators. Michael Montagne