Most people use a game engine the way most people use a car. They turn the key and drive. They have no idea what happens between pressing the gas and the wheels turning, and most of the time that is fine.
But the developers who understand what is under the hood are the ones who can fix it when it breaks, make it go faster, and build things the people who only know how to drive never could. The same is true for game engines. So let me pop the hood.
What a game engine actually is
Strip away the fancy editor and a game engine is a collection of systems that work together to do one thing on repeat: take the current state of the world, update it, and show it to the player. Over and over, dozens of times per second.
Here are the core systems inside basically every engine.
The game loop
This is the heartbeat. Every engine runs a loop that, roughly, does this every frame:
while (running) {
processInput(); // what did the player do?
update(deltaTime); // advance the world by one step
render(); // draw the new state to the screen
}That is it. That is the core of every game ever made, from Pong to the latest open-world monster. Everything else is detail hanging off this loop. deltaTime is how much real time passed since the last frame, which is how the game stays consistent whether you are running at 30 or 144 frames per second.
Entities and the things in your world
Your game is full of stuff. Players, enemies, bullets, pickups, doors. The engine needs a way to represent all of it as data it can update each frame. Early approaches used deep inheritance trees (a Goblin is an Enemy is a Character is an Actor) and they turned into a nightmare the moment you wanted a flying enemy that also talks.
Modern engines lean toward composition. Instead of "what is this thing," you ask "what does this thing have." This is the idea behind ECS, the Entity Component System.
An entity is just an ID. A component is a piece of data (a Position, a Health, an Inventory). A system is logic that runs over every entity with the components it cares about. The movement system grabs everything with Position and Velocity and moves it. The render system grabs everything with Position and Sprite and draws it.
This sounds abstract until you build it, and then it clicks: it is a clean, fast, flexible way to organize a world full of different things.
The event system
Things happen in games. An enemy dies, a player takes damage, a timer expires. You do not want every system constantly checking "did this happen yet?" Instead, engines use events. Something fires an event, and the systems that care about it react. It keeps the code decoupled and the world feeling alive and reactive.
The simulation layer
This is the math that makes a world feel real. How fast resources drain, how damage is calculated, how probabilities resolve, how things move and collide. It is mostly numbers and rules, and it is where a game goes from "technically working" to "actually fun."
Persistence
Saving and loading. Serializing the entire state of the world to a file and reading it back so the player's progress survives. Simple in concept, full of sharp edges in practice (versioning, what to save, how to restore it cleanly).
Why building one makes you better
You can use an engine for years and never understand any of this. The editor hides it all. And that is exactly the problem. When something is slow, or broken, or behaving weird, the developer who understands the loop, the entities, the events, and the simulation knows where to look. The one who only knows the editor is stuck guessing.
Building a small engine yourself, even a simple text-based one, forces you to actually understand every layer. You write the game loop, so you understand frames and timing. You build the entity system, so you understand how worlds are organized. You wire the events, the simulation, the save system. Suddenly every engine you touch makes sense, because you built the thing they are all variations of.
This is not theoretical for us. Inside Game Dev Junkies, students build a real game engine from scratch across the course, applying each concept as they learn it, and then ship an actual game on top of it. By the end you have not just learned C++ and game systems. You have built the engine and the game, and you can explain every layer, because you wrote them. That is the difference between someone who uses engines and someone who understands them.