Although it’s possible to inspect the code of an installed application, that’s not the kind of code you’re probably thinking of. In many cases, the code that you can inspect is code that’s very hard to understand. You can learn about the difference between “machine code” and “high-level programming languages” to understand why you probably won’t be able to do anything of value with the code that you can inspect.

Even if you can inspect it, you can’t change it. (You could, but it would be very very difficult.)

Even if you can change it, that doesn’t make it Open Source, for reasons that others have already described.

One place where you can often inspect the source code is in a web browser. That’s still not Open Source, but it’s closer to source code and largely available to inspect.

Best wishes with your interest in learning to program!