Custom faster-rcnn builder

Dear Support,
can you provide some insights in the following function:

builder

I want to implement my own builder to build a faster rcnn using my custom Resnet50. Maybe it is not enough to assign my resnet50 to the backbone of the faster rcnn as suggested in some posts elsewhere. I mean, the builder applies some processing to the original resnet50 I suppose I need to replicate them, is that correct?

Can you point to the strictly necessary lines of code within the builder I need to include?

I apologize for the topic maybe I failed to explain plainly.

Many thanks