It doesn't get carted away. Sometimes, they use worms, other times they use duff/leaves/whatever scattered into the mix. Maybe all of the above.
In the Rockies, they might sometimes have to carry it away on mules, but by then, it's just compost. On the AT, someone comes every once in a while and puts the toilet seat on the other of the two holes in the privy. So one batch of compost is setting up, while the other gets input.
In the backcountry, the preferred method for guys is to disperse pee and not use the composting toilets for it.
If you haven't used one yet, you'd be amazed how un-aromatic many of these are. Very clean.
There are many different kinds of these privies.
I found a photo of the kind with a great view. Again,
Rocky Mountain National Park: