One of the big advantages of Free an Open Source software is that you can study the code and change the code.

Many people attending my training classes who are not familiar with the open source development model yet are worried that if anyone can change the code then how can we trust the code?

I bet others who have not yet attended any of my classes also wonder about this.

The answer is simple.

There is a central place where the creator of the project (let's call her the "vendor") distributes the source code of the projects. Anyone can copy the source code from there and make local changes. Anyone can also re-distribute the changes version of the software under their own name, but only the vendor can change the code on the official distribution place of the project.

Open Source projects on GitHub

Every Open Source project on GitHub has an owner. Only the owner and maybe a few others the owner trusts can make changes to official repository of the project.

All the other people need to fork the repository, make changes on their own copy of the project and then the need to send a pull-request asking the owner to integrate the proposed changes into the official repository of the project.

Control

So the control remains with the owner and people the owner trusts.

Conclusion

Maybe instead of saying that "anyone can change the source code" a clearer phrasing would be "anyone can change a copy of the source code".