Hostile mobs have an infinite detection range.

1. Remember the coordinates of the spawn chunks
2. TP to a place far away from where you first spawned
3. Summon a zombie in a walled area
4. Summon the Iron Golem from the outside and stick the fight with the zombies
5. Then teleport the iron Golem to the coordinates of the spawn chunks location you saw earlier

Even though they are separated by ten thousand coordinates, the aggro is not released

https://drive.google.com/file/d/1_9qBSaVqNpli6zu4HyzsOQHzsUzVm7bl/view?usp=sharing