Let me state, that coming from Simulink, you should be aware of a complete change of the modeling approach. The modeling approach followed by Simulink is called causal (also assignment-based). Simulink actually models signal flows (every block in the Simulink block diagram produces an output signal in response to an input signal). On the other hand, ModelingToolkit follows (similarly as (Open)Modelica and Modelica-inspired Julia package Modia) the so-called acausal (also equation-based) approach, where you do not have to classify the signals/variables as inputs or outputs. This turns out very useful for modeling complex interconnected systems. Well, for a couple of years even Simulink users can enjoy the latter modeling approach through the the extension library called Simscape. Besides possible ModelingToolkit examples and tutorials, some intro to (Open)Modelica or Modia can make your transition easier, because it is really a transion from one mental framework to another rather than just a switch from one software to another.