system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How closed-source applications can exist.

Shouldn't it be possible to reverse-engineer the source code of anything? How can computers execute code that they can't see?
gpt
Think of a program as being like a house. A house needs to be built from a blueprint so the people working on it know how to built it. However, even if they've seen a similar house before, they don't necessarily know how to build it just by looking at another one, even if it's exactly the same as the new one is supposed to be.

A source code is the blueprint of a program. When it goes through a process called "compiling", the computer turns the source code into an executable program much like how the construction workers change the blueprints into a house. However, the executable doesn't look just like the source code, as the source code is built to be a human readable code equivalent to what the computer can read while the executable is what it looks like after it is translated to the computer's own language. This makes it so other programmers don't know how to reproduce it just by looking at the executable program.

Reverse engineering is theoretically possible with anything, but it isn't always easy, as you are trying to build a blueprint based on looking at the final product. It's kind of like figuring out the instructions on a piece of furniture that you didn't get assembly instructions for: possible, but it'll take some time to figure it all out and you probably won't get every step worded exactly like the original instructions.