The key feature is that the steering has Ackerman geometry
This means that the steering arms coming off the back of the hubs are connected to the steering linkage at a point inboard and behind the pivot point of the hub. If you draw a line through these two pivot points for both wheels they will meet at or near the centre of the rear axle (although the exact location may vary to produce specific handling characteristics).
The effect of this geometry is that, when turning the inside wheel will turn through a greater angle than the outside, reflecting the fact that the inside wheel follows a smaller turn radius. In a basic Ackerman geometry lines extending the axis of the front stub axles will meet at the centre of the turning radius, although again this may vary to suit particular handling characteristics.
It looks like the steering is controlled by a bell crank linkage rather than rack and pinion, so you may have to make some measurements to work out the precise relationship between servo position and steering input.
In practice there are also a lot of other related factors which affect the relationship between steering input and the actual behaviour of the vehicle as the steering geometry is also affected by suspension movement and the tracking, caster and camber angles of the wheels as well as the dynamic behaviour of the vehicle as a whole. This is particularly the case with something like an RC car which tends to have low mass and high torque so there may well be significant steering effects from factors like bump steer and roll steer, how the differential distributes torque to the rear wheels (ie the amount of slip in the differential and rear wheels) and the dynamic behaviour of the front tyres.
