A lot could be said about this, as I think it's rather a blurry line. Generally, historians are supposed to be careful and thorough, open to peer review and revision. Journalists are more often working to a deadline, and more willing to get something written than immediately get it right, and they're usually also working with more recent events. It's normal for a historian to begin work with written sources, while it's more normal for a journalist to call up someone and talk to them. I think historians are also more content with complexity, while journalists tend to want to simplify for clarity and impact, and as a result journalists sometimes can breeze too lightly though the written sources. And they often can be subject to a presentist bias, that people in the past can be judged by modern values and standards. And while a lot could and ought to be said on behalf of it, I think the 1619 book suffers somewhat because of all this: it simplifies things for impact, and didn't go through real peer review ( the authors picked the peers they wanted to review it, which is not really peer review). It also very much writes-to-a-purpose, concluding that if you don't want Black reparations you have no excuse now that you've read the book. Few historians would dare make such a direct political demand within, and based on, their own publication.

That said, the journalistic approach has sometimes been extremely successful. A good example of that is Charles Mann's 1491. Pre-columbian history of the New World is complex, based a great deal on archaeology , and theories and narratives of it have been disputed and often revised. Mann essentially covered the story of the various scholars and archaeologists who have worked on it, and what they have all said. In doing that he was able to make something much more readable than a stack of disputatious research papers yet still illuminating and reasonably accurate. It's only shortcoming really is that it was not two books, never really getting enough into Western North American Native Nations ( though Mann felt that Calloway's One Vast Winter Count did that). Its true that it is popular history, but a scholarly history would have been much, much bigger, and for the average reader probably inaccessible. And this is not necessarily limited to history: you could say John McFee has taken the same approach to geology...though, I guess that's history, too.

You mention the 1619 project, so I am operating under the assumption you are asking about what the difference is between historians and journalists when they write history.

I am not sure there is a concise answer to this matter. As you mentioned, both search for facts and make an argument using those facts. The difference is that a professional historian generally writes history concerning a topic which they have spent years studying. They have spent hundreds of hours researching and keeping up with that fields historiography, while also learning about historical methodology. A journalist, however, might have none of this historical training, yet may still have the dedication and writing skills necessary to write history.

Journalists seem to generally write popular history, not rigorous academic history. That gets into another debate about whether popular history counts as true history, which I have listened to my peers argue about endlessly. The consensus opinion I have heard is popular history is fine, as it is generally the most history people encounter outside of school. However, popular history does often trade rigor for accessibility, and that can obviously distort reality.

Not all journalists write popular history, some write academic history very well, and the same is true for trained historians. It is hard to make generalizations, but some patterns to emerge. I have not looked into the 1619 project, so I cannot say for certain, but it might have something to do with making that history accessible and popular, rather than historical accurate.