The IsoDidact, after firing the Halo Array from the Ark's Citadel, stayed on the Ark for a few years to help prepare the reseeding of the galaxy. He made a stop at Earth with Chant-to-Green make sure that the Homo floresiensis were happy with their new location when humanity was reseeded. After that, well, we don't know. He and the other Forerunners went on the Great Journey. It's widely believed they left the Milky Way, however there are sources that have clarified that there were no Forerunners left in just the Orion Arm. That lore is old though, so it's unknown how canonical it still is.