No clue. We don't know how they were composed or published. We know Cicero read some of them at least. If it were meant for wider distribution, it's hard to imagine how. It seems a stretch to suppose they were read aloud to the masses in a contio or something. We have the speeches Cicero gave in contiones about Catiline. These are much shorter than Caesar's books in their current form.

Commentarii, as a genre, seem to have been a sort af bare bones account written by a participant that served as the source material for a "proper" historian to dress up later. Cicero thought that Caesar's books were too good for this, and one may suspect that Caesar was trying to do something else.

They may have been intended for the Senate, but I don't know of any comparable series of reports. Seems strange that both the books about Gaul and the civil war could have been meant for that purpose.

I think that's all we know about the publication and intended audience for those books.

The topic is beyond my expertise, but yesterday I read T. P. Wiseman’s review in the Times Literary Supplement (June 10) of Cynthia Damon’s new edition of the Bellum Civile and her accompanying commentary. He says (apparently reporting Damon’s argument):

At the time, [Caesar’s] intended audience was very different. He was writing for the Roman People, as their political champion against a domineering senatorial oligarchy. It was the People who had given him his command, in the teeth of fierce senatorial opposition, and it was to the People that he reported what he had done with it. His narrative would reach them not through the written word (books were luxury items beyond the means of ordinary citizens) but viva voce, read aloud in the Forum or the theatre. That was why he wrote these accounts in the third person, not “I” but “Caesar”, to save whoever read them the daunting responsibility of impersonating him.

Damon argues that each book of the work was meant as a single unit that could be read aloud. Wiseman, again summing up Damon, says that

All the Gallic War commentaries (except the last, a later addition by Aulus Hirtius) report a single year’s campaigning, evidently composed as soon as the legions were back in winter quarters and probably rushed to Rome in time to catch the first of the annual “theatre games” in early April, where a big popular audience could be counted on. Similarly, Book 3 of De bello ciuili tells the story of one momentous year (48 BC), culminating in the defeat and death of Pompey “the Great”.

As a follow up how do we know that Caesar actually wrote those books. In fact how do we determine the actual authorship of a piece of literature written thousands of years ago.