Training of Region Proposal Network (RPN)
The first answer in your commented link answers one point about how region proposals are selected. It is the Intersection Over Union (more formally the Jaccard Index) metric. So how much of your anchor overlaps the label. There is usually a lower limit set for this metric to then filter out all the useless proposals, and the remaining matches can be sorted, choosing the best.



I recommend reading through this excellently explained version of a proposal network - Mask-R-CNN (Masked Region-based CNN).
If you prefer looking at code, there is the full repo here, implemented in Keras/Tensorflow (there is also a PyTorch implementation linked somewhere).

There is even an explanatory Jupyter notebook, which might help make things click for you.