Anchor Generator in Faster R-CNN implementation

I was reading Faster R-CNN codes that can be find here. I don’t understand part of it that I explain it below.

if rpn_anchor_generator is None:
            anchor_sizes = ((32,), (64,), (128,), (256,), (512,))
            aspect_ratios = ((0.5, 1.0, 2.0),) * len(anchor_sizes)
            rpn_anchor_generator = AnchorGenerator(
                anchor_sizes, aspect_ratios
            )

This code is for anchor generator when argument in None, It with be default setting for anchor generator. why anchor sizes is ((32,), (64,), (128,), (256,), (512,)) ? what is it mean? is it mean backbone contains 5 set of output? What backbone is assumed?

 box_roi_pool = MultiScaleRoIAlign(
                featmap_names=[0, 1, 2, 3],
                output_size=7,
                sampling_ratio=2)

Why roi has featmap [0,1,2,3] ? What backbone is assumed? why it is different from anchor generator?