Systems

The internal subsystems that nginx is built from. The repo's source tree maps almost 1:1 onto this list — each system corresponds to a top-level directory in src/.

System	Code root	One-liner
Core utilities	src/core/	Pools, slabs, hashes, rbtree, radix, log, regex, time, string, resolver, proxy_protocol
Configuration system	src/core/ngx_conf_file.*, auto/	How nginx.conf and auto/configure produce a running cycle
Event loop	src/event/, src/event/modules/	Connection table, posted events, timers, OS readiness back-ends
Process model	src/os/unix/ngx_process*.c	Master/worker, signals, channel, binary upgrade
HTTP server	src/http/	The 11-phase request engine, upstream, file cache, HTTP/2, HTTP/3
Mail proxy	src/mail/	POP3, IMAP, SMTP proxies
Stream proxy	src/stream/	Generic TCP/UDP proxy + load balancing
OpenSSL integration	src/event/ngx_event_openssl*.c	TLS handshake, certificate caching, OCSP stapling, session tickets
QUIC transport	src/event/quic/	The hand-rolled QUIC transport that HTTP/3 runs on
Logging	src/core/ngx_log.c, syslog	Error log, debug log, access log, syslog
Resolver	src/core/ngx_resolver.*	Async DNS for upstream hostnames
OS abstraction	src/os/unix/, src/os/win32/	Per-OS implementations of file I/O, sockets, processes, atomics

If you're trying to understand a code path end-to-end, start with the HTTP server page (or Mail / Stream for those protocols), then drill into Event loop and OpenSSL as needed for the lower layers, and use Core utilities + Primitives as a reference. The Configuration system page is the bridge between nginx.conf directives and the C state they produce.