GetComponent<T>() gets a component from the same GameObject.

Rigidbody rb = GetComponent<Rigidbody>();

Store references instead of searching every frame:

private Rigidbody rb;

void Awake()
{
    rb = GetComponent<Rigidbody>();
}

Safer lookup:

if (TryGetComponent<Rigidbody>(out Rigidbody rb))
{
}

Related methods:

GetComponentInChildren<T>()
GetComponentInParent<T>()
GetComponents<T>()