According to "Notes on differentiable stacks" by Heinloth,

the classifying stack will also classify $G$-bundles on stacks. (Remark 2.13)

(Here $G$ is a Lie group.) My questions are:

(1) What is the precise statement? (The category of morphisms to the classifying stack $[pt/G]$ is equivalent to the category of principal $G$-bundles? If so, how do we define morphisms between principal $G$-bundles?)

(2) How do we prove the statement?

(3) Suppose we have a principal G-bundle over a differentiable stack $\mathcal{X}$. How do we construct a morphism from $\mathcal{X}$ to the classifying stack?

I would be most grateful if you could tell me any good references.