Take a sculptor who works in marble. His week goes like this: he finds good stone, he cuts it and hauls it back to the shop, he spends the next week or two shaping it, then he takes the finished piece to market and sells it.
Four different jobs hiding inside one "sculptor." Finding, cutting, shaping, selling. Each one takes inputs and turns them into outputs. That's all a function really is.
And here's what happens next. Finding good stone is its own skill. You have to know where to look and what to look for, and do it badly and everything downstream is worse, because a great sculptor working bad marble still makes a bad statue. So if the sculptor is smart, and demand is high enough, he stops quarrying his own stone and pays someone better at it. Now there are two businesses where there was one, and a supply chain between them.
Run that logic all the way out and you get every business you've ever seen. Sourcing, transformation, sales, and the money functions (accounting to track what came in and went out, finance to plan what comes next). A thousand-person company isn't doing something different from the sculptor. It's doing the same handful of jobs, broken down further and further to handle the scale.
This is why I look at businesses the way I do. From the outside, a business isn't a black box. It's a stack of functions. Some of them are the reason it exists, and some are just there because nobody has outsourced them yet. Telling those two apart is most of the work.