In the early 2000s, we used to write a lot of CGI programs.
This was the primary way to make websites dynamic at the time. These CGI programs were usually written in Perl, but sometimes in C to increase performance.
The CGI mechanism is conceptually simple but powerful. When the web server receives an incoming request handled by a CGI script (e.g. GET /~jakegold/cgi-bin/guestbook.cgi), it:

Sets up environment variables containing request metadata (HTTP headers, query parameters, request method, etc.)
Spawns a new process to execute the CGI program
Passes the request body (if any) to the program via stdin
Captures the program’s stdout as the HTTP response
Sends any error output from stderr to the error log

The CGI program reads the environment variables to understand the request, processes it, and writes an HTTP response to stdout, starting with headers.

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A performance analysis demonstrating that CGI applications can handle impressive loads on modern hardware, achieving over 2400 requests per second or 200+ million requests per day. The author benchmarks a simple Go-based guestbook CGI program using both Apache and a custom Go HTTP server, showing that while CGI may not be the optimal choice today, it remains surprisingly viable for high-traffic applications when paired with contemporary multi-core processors.

Serving 200 million requests per day with a cgi-bin