Is there any code for Mobile-First Indexing in HTML?

Yesterday, I went for an interview and they asked me what is mobile-first indexing and I replied that creating the website or structure of website by keeping mobile in your mind means you create website for mobile as google give preference to mobile.

But, the interviewer said there should be some code for mobile first indexing but I don't know any such code. Is there any code exist? I am pretty sure not.

Please let me know if I am wrong