How does developers write code and work together in big companies?

I wonder how developers in big or even 10 employee small software companies write code together. I don't think every employee getting the soruce code of the product of course for security reasons. But without getting the soruce code I really can't imagine how an employe work and do what is wanted in the project. For example if a developres task is building a chat application for a webapp, how can the developer do that without getting the soruce code of the website or reaching the database of the website?