After running, the planet transforms into the core. You may need to queue its upgrades via the normal Giga interface.
Instead of waiting 30 years for a Matrioshka Brain to boot up, use these instant-effect commands. stellaris gigastructural engineering console commands link
Inactive Black Hole (for the Birch World): effect set_global_flag = giga_core_birch . After running, the planet transforms into the core