Nearly the entire Google codebase is accessible to any engineer at the company. Engineers can look at code from completely unrelated projects within the company. Code transparency is a powerful concept that drives culture and knowledge sharing within the organization.
Culture. Code transparency is part of the culture. First, there's an element of trust. Second, it enables new developer workflows. Google has a concept called 'readability' for each programming language. These readability experts check for style and programming language usage. Not only domain experts but also programming language experts have to review submitted code.
Knowledge sharing. Siloed code prevents knowledge sharing in companies. Developers learn best by looking at code. While I was at Google, I spent hours looking at code examples that I known had been battle-tested and perfected. I believe that made me a better programmer, even if the domain was significantly different.
Practicality. Code transparency is practical. Putting code in silos and adding authorization requires extra effort. Developers who create internal libraries may not understand the full extent of the library's usage. Practically, developers of libraries can take ownership of version updates by fanning out updates to all different projects.