Writing my own Custom NPC

Looking for any help getting started on creating my own custom NPCs with their own behaviors and what not. I am having trouble finding documentation or tutorials as they tend to reference the in-game command block or existing mods that contain their own custom-made NPC. To be clear, I am looking to write my own. I decided that in order to touch up on my Java, I'd have some fun writing some code for the game. But I can't seem to locate exactly how to manipulate the NPCs (movement, functions, etc.). I'd appreciate if someone could point me in the right direction on how I can approach this, that would be great.

Thanks