Primitive colliders are the most computationally efficient type of collider in Unity. They are called “primitive” because they are defined by simple geometric shapes such as boxes, spheres, and capsules. They match the Primitive Objects, which are built-in GameObject shapes.
There are three primitive collider shapes in Unity:
Primitive colliders are efficient, but they have limitations. For example, you cannot change or deform their shape, only their scale. Unlike Mesh colliders, they are not made up of triangles; their shape is fixed (note that the Physics Debug visualization does indicate a triangle-based mesh on primitive colliders, but these are for visualization purposes only and do not reflect the collider’s construction).
Primitive colliders are usually not the best option for complex shapes, wheels, or Terrain collision. Unity has specific colliders available for more complex collider shapes (see Mesh colliders, Terrain colliders, and Wheel colliders). However, if you need a collider shape that fits a complex shape, but does not need to be too accurate, you can use primitive colliders to create a Compound collider. A Compound collider is a collection of primitive colliders in an arrangement that fits the collider shape you need.
In summary, primitive colliders are an efficient but sometimes inaccurate way to add collision detection to objects in Unity. They are suitable for primitive-shaped GameObjects with regular shapes. However, for more complex objects with irregular shapes, or for more accurate collision detection, you should use a more complex collider shape.