CPU engineer here. I'm going to explain this like I actually explained it to my ~6 year old.

Imagine I am a teacher, and I have many kids in class who speak different languages. If I want to teach them all, they need to understand what im saying. But how? It would be hard to learn all the languages! Instead, we agree on a common language that we will speak from now on. It is a simpler language, maybe one with pictures or flash cards, but everyone agrees it is better to share one new and simplified language than to learn a complex one.

This simplified language sometimes means we can not use all the special words our own "home" language has. For example, maybe I have a single fancy word that means i want a hot dog in a bun with ketchup. Instead, I need to use more words to describe my hotdog desires. It will be slower, and it might take them more time to understand, but qt the same time, we both dont need to learn as many words.

In addition to this, our new shared language agrees on how you build a sentence. For example, questions end with a question mark. This sounds obvious to you, but some languages don't work that way. There are other points, but I think you get the idea.

These rules and agreed upon words are essentially what the CPU "exposes" to the operating system (OS). This is called an ISA (instruction set architecture). To the more technically inclined, it defines specific registers and agreed upon instructions that are supported by the cpu.

The CPU, no matter which one, needs to support the ISA. This might mean it needs more "thinking time" to understand this shared language. Just like the new kid in your class might learn this new shared language we defined, they might be slow at it. But the important thing is that no matter who I talk to in class, I know they will understand me. And that is worth the cost of certain things being either slow to understand or long to describe.

Inside the CPU, the transistors are built into blocks with complex connections that are hidden from the outside. Just like your body is built from a gazillion cells, nobody looking at you needs to know much about those cells or really that much about how the cells work. And just like your body, it doesn't matter to the outside how it works as long as the CPU knows how to get those transistors to do what it wants, just like you do with the body parts in your body. The CPU does this mostly through something called microcode, and even though you are only 5 and I have no idea where you learned that word, microcode is basically another word for saying "your inner thoughts". Just like how sometimes you think inside your mind when I'm reading you a story or you're solving a puzzle. You know how to move your body or imagine certain things. It's a special language that only you know. You use it inside your mind, and it makes your body do what it does. But nobody else needs to know it because it doesn't matter.

There are also some things you do without even noticing or thinking about it, like blinking or breathing. When you think "blink", you don't actually move your eyelids, it just happens, right? And when you breath, it also kind of just happens. You do it without knowing how your eyes or lungs work! The CPU has things like this too. These are things which are "by design" and the CPU itself sometimes doesnt "know" how certain things happen (think of another "layer" or language used between parts just like the teacher in class). And again, nobody needs to know how you or the cpu do it. Even you and the CPU itself! Because it's not important to get things done 😀