Cowboy is an open-source, small, fast, and modular HTTP server designed to handle a large number of simultaneous connections efficiently. Built on top of the Erlang programming language, it provides a feature-rich and lightweight web server that is optimized for low latency and high concurrency. Frequently used in real-time web applications, Cowboy offers support for WebSockets, HTTP/2, and RESTful services.

NGINX is an open-source project that provides different kinds of technologies to help developers create complex and sophisticated applications with ease. It comes with a range of multi-cloud application services to help developers fulfil their requirements. 
NGINX is commonly used by users to build microservices. It also offers the NGINX Partner Program to help users gain access to and grow in a set of skills and capabilities. It provides users with the ease of deployment on any cloud. It also helps by simplifying the entire process and eliminating vendor lock-in. NGINX is mainly recognized for its high-quality security and documentation. NGINX provides NGINX controller, App Protect and Unit to its customers.

Comparison Cowboy vs Nginx