Backpropagation in Faster R-CNN
Where exactly are the weights updated? Is it between the initial convNet and the RPN. All updateable weights are updated during backpropagation.

I assume you were trying to ask when are the weights updated?, well according to the original paper there are 3 way to train Faster R-CNN, each has it's own when to update weight time:


Alternating training aka 4-step alternating training, which is the one used by the author in the original faster r-cnn paper. It's explained quite well in the original paper
Approximate joint training, the one you mention in the question, after a full forward pass of both networks, a backward pass is done. I'm not quite sure what you mean by "In this case how are both considered simultaneously?"
Non-approximate joint training, I don't really understand about this one


Back to the Where exactly are the weights updated? question, well, some to be mention are the RPN kernerls, RPN Cls and Reg FC Layer, the detection (Fast R-CNN) Cls and Reg FC layer also the CNN kernels (if you didn't freeze them)

I hope this help since I'm also learning about Faster R-CNN too, also sorry for the bad English.