Communication between Keras and Backend like Tensorflow, Theano

Question

Keras is a high level neural network API providing python library which uses tensor flow or theano or cntk as backend. What are the primary roles of backend libraries? Is it implementation? or Is it computational heavylifting using GPU, threading etc?

I couldn't find any good resources online to understand how keras interacts with tensor flow or theano(backend) session. Any such resource or direction for understanding interaction is helpful !