without getting into some of the loaded terms of 'real-time' and 'HIL' and what that means to folks...
Assume you want to put algorithms on some target processor (you call out that you have some arduinos), and you want that platform to execute that algorithm in 'real-time' while talking to hardware (potentially both inputs and outputs)...
There is not RTOS running on the arduino, but there is an overrun flag that flips a bit in case the processor isn't keeping up, so at least you know it isn't keeping up and try to change something. You will get more fidelity/capability as you move to RTWT, xPC Target, and then potentially other embedded platforms. It is true that xPC Target is the support from MathWorks for full featured and commercial strength offerings, especially when paired with Speedgoat hardware to form xPC Target Turnkey. There are also many third party options (and you can look at the connections program for those).
Lastly generally the application and requirements drive the real-time and HIL solution needed, which would likely push you in one of the directions that folks have suggested.
Good luck and have fun learning!
HTH Dan
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