Calculator app using Scss

One question I have is the structure of your scripts. I'm used to programming in Java and C++/C. In those languages you have a main function that calls everything else. I'm still figuring out how to be more organized with my script. I've seen some js people make a main function that calls everything else. Others use more complicated modular features of the language that calls everything else. Any readings or advice you might have on how to better structure my js is appreciated.

The other question is the use of the DOMContentLoaded event. I remember some other tutorials would put their entire script inside the callback for this event. I don't think that matters if one uses the defer keyword in tags in the HTML file using the script. Is that true?