Whirly
IMO, not "what", but "when" and "how."
The PPL syllabus pretty much ends at PPL - where is the follow on syllabus for more experienced PPLs?
Airline pilots have recurrent training in a sim environment, lots of learning there (I'm lucky I had 15 hours with a TRE in a sim and learned a huge amount about flying, even though I will never be competent to fly the arcraft type the sim was simming)