-- There doesn't seem to be a good amount of plumbing on the side of Arrakis where the Natives live. There were a lot of people in those caves. Where are they all shitting?! And without trees, it's rather difficult to make toilet paper. Not sure how the natives are handling that part of it either.
-- So the Natives live in a humid desert but they don't have running water?! They NEVER shower?! And Willy Wonka banged that one girl?! Grosssss!! There was like 20 years of musk underneath all those clothes!
Gross! Wonka had sex with that "un-showered for decades" girl!
Even if never showering were remotely possible for a race of people, and it isn't, where are they crapping?! Where's the sewer system?! Any sewer system would need water. And before you go here...Covering crap in sand isn't the same as burying it in soil, it doesn't deteriorate well in sand without being exposed to the sun. There seemed to be millions of people in that final scene, hundreds of thousands at the very least...that would be a lot of crap-covering without much deterioration! The sand surrounding the area where these millions of people lived would get really nasty really fast. And no water to wash your a-hole?! Oh man, that poorly-washed area wouldn't stay very healthy for long! There would be no reality where a race of millions of people like this could exist without plenty of drinking/cooking/cleaning/bathing water. There are desert-dwellers/nomads here on Earth...but they all have access to water to drink and clean themselves at least occasionally. And since there aren't millions of them, their feces is spread out over vast expanses, not a problem.
They crap in their suits which process feces into base materials to be reused. At least thats how the book describes it, the movie just ignores this.
There are not millions of people in any of the scenes, some scenes may include thousands of CGI renders, but with real extras i doubt they ever had even a hundred in one scene. Altrought lore-wise there are apperently a billion people living in sand caves.
To quote the author of the book, it does not need to make sense. All technology exists solely to advance the plot being delivered and does not consider any scrutiny being applied to it. Dune never made sense to begin with.
P.S. interesting fact: Muslims, which the Fremen are heavily based on, have it in quaran that if you cannot find water to wash yourself before the prayer you are supposed to rub yourself clean with sand.