Don't want to write this by hand? GodCraft sets minecraft:memory_behavior.follow_target for you with a switch.Get GodCraftOverview
minecraft:memory_behavior.follow_target is an entity component: it goes in the components object of a custom mob, or in one of its component groups. An AI behavior that uses the entity's memory to follow a remembered target. It has little public documentation, so test it carefully.
Example
Inside the components object of BP/entities/<name>.json (values are the defaults or placeholders):
BP/entities/<name>.json
{
"minecraft:memory_behavior.follow_target": {}
}