WordPress is built by volunteers. People from all over the world collaborate to create the core software, to write the documentation, to provide support, to translate WordPress, to organise events, and to generally keep the project running. Individuals work on WordPress in their free time and companies ask their employees to get involved.
A bunch of WordPress contributors.
Part of WordPress's success is that it is not simply a development community. There are designers, user experience experts, support volunteers, writers, users, accessibility experts, and enthusiasts. This diverse input strengthens the project. It also means that there is space for you to get involved. Whatever your skill set, there is room for you in the WordPress community.