why transform is accessible in C# directly buy other components aren’t accessible directly and have to get them?
Because there is always a transform on every GameObject and the transform u can access comes from the Component class which in inherited by the Behaviour class which itself is inherited by the MonoBehaviour class from which every unity C# script derives from, there used to be before Unity 5 other properties like: audio, rigidbody, light, etc. but have been deprecated
You are probably referring to the module using Unity individually.
As the documentation states, each object in the scene has a Transform component provided. So when for example you call a code that does not refer to another instance you refer directly to a gameobject and get all its properties including mainly the transform.