How are True Negative and False Negative converted into True Positive and False Positive in ROC curve? I found the ROC explanation at this link.  It states that the ROC curve is TP vs FP.
After the score has gone below 0.5, all predictions are negative.  That makes them either TN or FN.  Thus, how does it make sense to continue drawing as the axes are stated as TP and FP?  I am really confused.

you vary the threshold which is used to determine positives and negatives, ie at threshold 0.5 we get a point (TP, FP)  which we plot, then at threshold 0.45 we get another point (TP, FP) which we plot, etc. These points make the ROC curve
