ringback is only generated by the originate process when an a leg is supplied.This means when you run the bridge app.
ringback is used on unanswered calls and transfer_ringback is used on answered calls unless its blank then ringback is used.
originate, by default terminates when media is reached (aka 183) if {ignore_early_media=true} is specified in the dial string, then it will remain in the originate state until 200ok is received.
if ringback var is set, as soon as the b leg sends in ringing indication 180/183 the data from that variable will be played to the inbound leg or A leg.
The hold_music is used by the system for any other time something wants to send music while busy.