I know three.js can be extended and customized. I’m wondering if there is a document or repo that shows how to accomplish certain functionality in three.js that Unity has built-in. If not, has anyone here had experience seeing a type of functionality in Unity and replicating it for three.js?
I don’t think that this scales well. With unity’s asset store, any asset is a candidate to be used for such an example.
This is a very broad question — Unity has a lot of features, and even more including the asset store. Some are easy to reproduce, some are very difficult. three.js is not a game engine, although other projects like Yuka and Cannon.js can be used to add some of the functionality you might need when building a game.
Did you have a particular feature of Unity in mind?