How to correctly spread functions across files?

Question

Programming

To begin with, I am not a programmer, I rarely program, for myself and usually the goal is to work, and not to work well and efficiently.
One of the projects has now begun to grow and there is a need to spread everything into different functions, files, folders, etc.
Surely programmers have come up with a bunch of cool schemes and tricks for a long time, how to do it correctly, so that later it would be convenient to poke around. Can someone suggest a good article, diagram or something. Like as it is PEP8 - a set of generally accepted rules of good taste how to structure the code, how to name functions and variables, etc., but only about the architecture?
Preferably not a book, so that reading and understanding it takes no more time than developing this small project.

Answer 1

PSR-4 is about files and autoloading, PSR-2 is about what's inside a file.

Answer 2

Context of meaning, you can read about DDD