What means the comparison between empty parantheses without a function and content of curly brackets

Question

I'm working on a code right now and in this code are some parts which I not understand. Especially the setInterval part in which are empty parentheses and comparing with the comparing operator => with the content of the curly braces. Further if the querySelector is better than defining an id and access the html elements with getElementById. Lastly I want to know if the onclick method which is not in HTML part but in the javascript part is a implemented method of javascript

const s = document.querySelector('span');
const b = document.querySelector('button');

let interval = null;
b.onclick = () => {
  // Start the interval
  if (!interval) {
    interval = setInterval(() => {
      s.innerText = parseInt(s.innerText) - 1;
    }, 1000);

    b.innerText = "Pause";
  } else {
    clearInterval(interval);
    interval = null;

    b.innerText = "Resume";
  }
}

<span>100</span>
<button>start</button>