You don't need that knowledge as i just went to GPT and asked it ...

"I need to create a dropdown for a website can you help me make it?"

And then I asked,

"How do I make what you wrote above work?"

It detailed the things one needs to do ..copy/paste each block of code in three separate notepad files and save each one accordingly (index.html, style.css and script.js) all in one folder. Once that's done double click on the index.html to run the dropdown.