How can different dev code for one app?

I’m learning web dev now and it’s easy to imagine how one person can do a single site oneself. But I’m wondering in a bigger project such as Facebook or desktop apps or even phone apps, how can or how do different developers work on different parts of an app and how is it pieced back together? This fascinates me. I’m imagining different files for different parts of the app which are linked together? And each developer is in charge of coding each component? Thanks.