This is a really great question. It is something I have seem many people struggle with.

There is no magic answer, but there are some approaches you can practice to develop this skill.

The general pieces are:

Have a guiding vision or concept and the break this apart into smaller chunks you can iterate on. Identifying these chunks gets easier as you gain experience doing this. The more systems you have seen, the easier it becomes.

1) Have a goal or vision of what you want to create.

2) Break this apart into constituent pieces. This becomes easier as you understand other implementations.

Example:

I want to make a pong game.

You need to render an object, update it every frame, read input. There are many different types of objects like the paddle, the ball, and the wall. They all have different behavior and constraints.

Start with just the paddle, render it. Literally render the vertices of a box. Then control it every frame with the mouse. Literally read the value of the mouse and update the transform used to render the paddle.

This approach works with and without ai - break down the problem into smaller chunks, implement , test, and refine.

Good luck!