I am an initiative coordinator on Drupal 9, here is my writing on tools and processes getting there.
Join the Drupal Community in this worldwide event focusing on Drupal 9 stability and adoption across contributed projects. Over 1600 projects are already Drupal 9 compatible a month before Drupal 9's release which is unprecedented. However, there are still thousands that only need very small changes and a release made.
The first Drupal 9 Porting Day was on April 28, 2020, led by Gábor Hojtsy, Lee Rowlands, Vladimir Roudakov, QED42, Srijan, Adam Bergstein and Mike Lutz. Altogether 126 issues were worked on that resulted in 89 newly Drupal 9 compatible releases on that day and the following two days. It was not only successful but we had a lot of fun too. So we are of course here to do another one this month!