1. Some people have mentioned it already but turning human-readable code to machine code is like turning a precise cake recipe into a cake. A decent baker can do it. Any developer can compile existing code. Going from a baked cake to a precise recipe requires a chemistry lab and a team of highly-trained scientists. Same thing for code. You’ll need a highly-experienced and specialized programmer to turn a program into legible code. Its almost always easier to just rebuild from scratch.

However companies do other things as well.

2. Companies sometimes purposefully compress and obfuscate their code to make it hard to unpack. This happens a lot on the web where a website might have code sent to your machine in a format which could have been legible. But before they send it to you, they run the code through a program which adds extra steps, renames things, and reorders things and removes extra spaces… all to make it hard to read.

3. Some companies will encrypt their code or programs to varying degrees. Some will do it at the storage level, such as DRM or modern disk-based videogames. The data in these games is “locked” behind passwords and keys which can only work if the program “calls home” to Steam or Xbox or whatever and those providers let the game be opened. It’s more complicated than this but that’s the basics.

4. A lot of companies have moved their code “into the cloud”. That means, instead of giving you a full piece of software, you only get the front-end, or the pictures and words you see on screen. The actual program lives on the company’s servers which you don’ have access to. You only get to send those servers inputs, and they return outputs back to your screen.

5. Companies can make their code secret from internal developers by breaking programs up into smaller pieces. Say you’re a developer at Apple. You might be assigned on the specific part of the system which opens apps from the home screen and may only get access to that part of the system so if your development machine gets hacked, the hackers don’t know ALL the inner workings of iOS.

I’m sure there are more ways but this is a start.

People here talking about decompiling code. Maybe you meant to ask how they’re certain it’s not stolen from home base?

All our goods are hidden behind a VPN or MFA (multi-factor authentication). Unless someone does something stupid, you’re not getting in our BitBucket or AWS account.

A lot of iOS is open source.

But to actually answer your question, most companies of a big enough size will issue the devices their workers use to them, that way they own the machine the source code will be on, rather than the employee. They will also have some sort of enterprise management software on it that gives them remote administrator access to delete everything in case the device is stolen.

As for disk encryption, they use the same disk encryption methods consumers use.

But sometimes source code does leak. Either accidentally or maliciously. Often times, that source code isn’t really useful to anyone else. Like when Facebook’s source code leaked, it wasn’t really that big of a deal, because Facebook is the only company that has the tech stack in place to run it. And it’s not like Google is gonna blatantly steal Facebook’s code.

The biggest risk to leaked code like that is that someone will discover a vulnerability in it and use that to compromise the company’s service.

Often times the employee (or ex employee) who leaked the code can be found and sued or prosecuted. It’s an incredibly stupid thing to leak that kind of code, because it really doesn’t gain you much if anything, and puts you in huge legal jeopardy.

Source: I’ve worked as a software engineer for Facebook, Google, and Microsoft.