Main technical features and functionalities
Simulink is a programming environment integrated in MATLAB and used for simulation and development of complex systems from concept to implementation. Although the software is not free, MATLAB is widely used in engineering courses and a lot of universities have campus license, Simulink included. Due to popularity of MATLAB, Simulink is fitted to run in Windows, Mac, Linux with enough disk space and RAM on the general computer.
The models are designed using block diagrams of the specific domains. Simulink suggest various templates to help designing the model and many examples. However, the software is dedicated for the professional use and requires specific knowledge.

Examples on how to use them to analyse Complex Systems
Simulink can be used to simulate and analyze multidomain complex dynamic systems, such as system for wireless communications, power electronics, signal processing, robotics, driver assistance, artificial intelligence, and others. Simulink enables to test design ideas at the early stage, simulate large-scale models and reuse components, validate the requirement set and system architecture under various conditions. The Simulink Help center provides a large number of examples, including modeling an automatic transmission controller, automatic climate control systems, air traffic control radar, elevator system, fitness tracker, and many others. Examples can be found in: http://www.mathworks.com/help/examples.html
Also, Simulink is used in scientific research to model complex systems:
doi.org/10.1088/1402-4896/abdf0c
doi.org/10.1016/j.compchemeng.2019.03.009