Nginx is developed by a commercial company having very highly qualified full-time paid software engineers. At the same time, the open source version of Nginx, which covers 99% of your needs, is always available both as a collection of versioned tarballs and as a Mercurial repository with commits history under a very permissive BSD license that allows modification and redistribution. This is a unique opportunity to learn from a corpus of very effective and professional C code. Nginx supports modules, and there are numerous third-party Nginx modules available (see the official wiki page https://www.nginx.com/resources/wiki/modules/). Nginx module development is the main way to customize the software for specific needs and is a very valuable skill that you may build your career on.
Nginx is written in C language with very little external dependencies. It contains its own highly effective library of primitive data structures and algorithms, such as hash tables, radix trees...