Does Avalokiteshvara speak through the Buddha in the Heart Sutra? Or how is it that Avalokiteshvara speaks to Shariputra?

I wanted to know how it is that Avalokiteshvara answered Shariputra in the Heart Sutra. I thought Avalokiteshvara was a cosmic being representing the compassion of all Buddhas. As such he would not be a person who found himself in the Buddha's presence as He delivered his Dhamma discourses, right? So, does Avalokiteshvara speak through the power of the Buddha or is just that Avalokiteshvara's compassion is "part" of the Buddha since he is indeed fully enlightened? Who is it that answers Shariputra? Is the one abiding in the Prajnaparimita Gotama Buddha? I don't really understand.