how do multiple developers work together to make something?

Me and 2 of my friends are planning to build a small little 2D physics engine for fun and I wanted to know how people work together to make a piece of software. like for example let's say you have 4 different pieces that you have to code up and 4 different people do it. but how do they integrate those 4 different modules into one application? Let's say one particular module depends on another one's variables and functions how does the person know what variable and function names the other person is using? how do we work together on this?