I think the most important point, which keeps being stressed then ignored, is that THE RA MAY NOT BE CAUSED BY THE AIRCRAFT YOU CAN SEE.
In RAT 5’s example above, it would have been easy to fixate on the aircraft in front (not implying you were, RAT 5) especially if you were already getting a TA. When it turns into an RA it could be another aircraft attacking from the side that you weren’t previously aware of. I actually had this exact scenario happen at JFK on long final and it was VFR traffic that wasn’t talking to Kennedy...