The market demo selected all .stall elements and used forEach to update each one. Your trail page has three .stop list items. Each should gain the suffix ** — marked**. The CSS already styles the list; your job is in script.js.
The same shape works on a different page, a craft fair:
const booths = document.querySelectorAll(".booth");
booths.forEach((booth) => {
booth.textContent += " — checked in";
});
The selector produces a NodeList; each callback receives one element. Do not write to the NodeList as if it were one element, and do not use querySelector, which would change only the first stop. Run to see three marked stops, then Submit.
querySelectorAll(".stop") and forEach to append — marked to the text of each of the three trail stops.