How to make a military unit able to harvest resources?

[Cactorum Rex]

I want to make a ship that is able to harvest a specific resource, but I cannot find anything related to harvesting resources in the data directory of the game. The improvements file seemed like it was on the right track, but there was nothing about harvesting. Can someone tell me where I can find the base game stuff about harvesting, or how I can implement a system like this?