You're updating the entire innerHTML of cartProduct for every iteration inside your for-loop. That's why only the last item gets put on the DOM.

Create an empty string outside the for-loop, and append HTML to that string for every iteration. Then update the innerHTML after your for-loop.

function checkStorage() {
  let cartContents = document.getElementsByClassName("products")[0];
  let cartProduct = document.createElement("div");
  cartProduct.classList.add("product");
  let cartCheck = JSON.parse(localStorage.getItem("localItem"));
  // return cartCheck;
  savedCart = "";
  if (cartCheck === null) {
    return;
  } else {
    console.log(cartCheck);
    let savedCartsHtml = '';
    for (const check in cartCheck) {
      if (Object.hasOwnProperty.call(cartCheck, check)) {
        const element = cartCheck[check];
        savedCart = `<img width="60" src="${element.img}" alt="iphone" />
                    <div>
                      <span class='title'> ${element.title}</span>
                      <div class='price-section'>
                      <input type="number" value="1" class="cart-quantity-input" />
                      <span class='cart-price'>${element.price}</span>
                      </div>
                  
                    </div>
                    <button class='btn-danger'>
                      <i class="bi bi-x"></i>
                    </button>`;
        savedCartsHtml += savedCart;
        console.log(cartProduct);
        cartContents.append(cartProduct);
      }
    }
    cartProduct.innerHTML = savedCartsHtml;
  }
}