The paragraph is the most indistinctly defined unit in all of writing theory. In the 19th century it was common for paragraph to run on of a page or two. Today, they often run only a few lines.
For instance, the paragraph break I just made does not really indicate a shift of thought or action or theme. There is no hard and fast reason that I can think of that would justify breaking it or not breaking it as I did. The main reason for the break it that it seems to give the reader a bit a of rest and make the text a little less daunting to read.
Why less daunting? It is unclear, at least to me. The thread of argument is the same as it would be if this were still part of the first paragraph (as it certainly would be in 19th and most of 20th century prose). The breaks don't make the argument any easier to understand. And yet somehow this answer will be perceived as easier to read for being in three paragraph rather than one.
All of which it so say that in many cases the focus does not shift between paragraphs. A whole sequence of paragraphs will generally have the same focus, and probably should.
In fiction, it takes time for a reader to settle into a scene. You have to build up a mental picture of the setting and the characters and the action and follow it through to a logical conclusion. A shift of focus is mentally taxing for the reader. Any text that shifts focus rapidly is going to be hard to follow for no other reason than that rapid shifts of focus are exhausting for the reader under the best of conditions.
Remember that prose is an asynchronous medium. Things do not happen in real time. Often an event takes far longer to describe than the event would have taken to occur, or, vice versa, far longer to happen than to describe. For this same reason, handling events that happen simultaneously by rapidly switching back and forth is usually a bad idea in prose. (It can work very well in movies, where the camera does most of the imaginative work for the viewer.)
So, first and foremost, the way to make writing flow fluidly is to maintain focus on a single scene or sequence for as long as you reasonably can. Shifts of focus are inherently taxing on the reader, so keep them to a minimum. There is nothing wrong in a book in sending Jack off on 80 pages of adventure, and then saying "As soon as Jack left, Jill went to fetch a pail of water". That is hard to do in the movies because they are a synchronous media and people expect the time sequence to be inviolate. It is hard to insert this simple bit of narrative in a movie. But in prose it is easy to do, and generally preferable, since it helps keep each narrative thread flowing smoothly.