If ... find next ... and copy to new column without blanks

Hi guys,
I have several hundred .txt files which each contain thousands of rows. What I need to do is find rows which start with a certain string ("Details" *) and copy this to a new column without blanks. The added difficulty is that I also need to copy the first instance of "Onset" * after this indicator (variable # of rows between them) and also copy this to a new column without blanks. See attached for an example what I would like to be the outcome.
Any and all suggestions are very much appreciated.
Kind regards, Dennis

p.s. Bonus problem! Is there a way to open all the .txt files and run the solution for the problem above on all of them?