iTween uses C#, and I know that C# can only run on a Windows OS, and not in a web browser, but will using iTween prevent me from making a game in JS and have it run on Mac/Linux OS’s and in web browsers? I know this seems like a dumb question, but I don’t know how Unity handles this.
“I know that C# can only run on a Windows OS, and not in a web browser”
Incorrect. Not sure where you get that from.
Your Unity C# scripts will work the same on all supported platforms.