Selecting and updating elements

const heading = document.querySelector("h1");
const cards = document.querySelectorAll(".card");

heading.textContent = "Updated!";
heading.style.color = "navy";
heading.classList.add("highlight");

querySelector returns the first match using a CSS-style selector; querySelectorAll returns all matches as a NodeList you can loop over with forEach:

cards.forEach(card => card.classList.add("visible"));

Create new elements and add them to the page:

const li = document.createElement("li");
li.textContent = "New item";
document.querySelector("ul").appendChild(li);