Ask HN: How to effectively transfer knowledge from senior devs to junior devs

The organization that I work for is in a bit of an unusual situation. We have a group of core developers (three people) who have been on this project for over twenty years and a group of junior developers (six people) who have been on the project for less than three years (most less than one year). There's no one in between those two groups.

The project itself has been ongoing for over thirty years, and we are struggling to transfer knowledge of the intricacies of the project from the seniors to the juniors. Does anyone have any recommendations for an efficient way to do this?