How does a Camera move freely inside a car in a movie scene?

For Children Of Men, a film that features several lengthy single-shot sequences, they built an extensive rig on top of the car, and also modified the car seats so the actors that weren't on screen could get out of the way of the camera.

According to the movie's Wikipedia page:

Cuarón's initial idea for maintaining continuity during the roadside ambush scene was dismissed by production experts as an "impossible shot to do". Fresh from the visual effects-laden Harry Potter and the Prisoner of Azkaban, Cuarón suggested using computer-generated imagery to film the scene. Lubezki refused to allow it, reminding the director that they had intended to make a film akin to a "raw documentary". Instead, a special camera rig invented by Gary Thieltges of Doggicam Systems was employed, allowing Cuarón to develop the scene as one extended shot. A vehicle was modified to enable seats to tilt and lower actors out of the way of the camera, and the windshield was designed to tilt out of the way to allow camera movement in and out through the front windscreen. A crew of four, including the director of photography and camera operator, rode on the roof.

This is of course best explained visually, and they did so in one of the extras on the DVD/Blu-ray called "Under Attack":

But these single shot action scenes are still composites of multiple shots. Again citing the movie's Wikipedia page:

However, the commonly reported statement that the action scenes are continuous shots is not entirely true. Visual effects supervisor Frazer Churchill explains that the effects team had to "combine several takes to create impossibly long shots", where their job was to "create the illusion of a continuous camera move." Once the team was able to create a "seamless blend", they would move on to the next shot.

[...]

the car ambush was shot in "six sections and at four different locations over one week and required five seamless digital transitions"