After adding the product to cart, open View and Edit cart page in the frontend.

In the cart page one text area will appear for each product.

Enter your comment there, and then checkout.

You can see comments for each product in the Sales > Orders > Edit Order under items ordered grid.

After adding comments & placing the order, customers can see their comments in the “My Orders” section.

Comments are visible in email for customers

Customers can see the cart comments in the order confirmation email for customers.

You can see comments for each product in the Sales > Orders > Invoices under items invoiced section.

You can see comments for each product in the Sales > Orders > Shipments under items shipped section.

You can see comments for each product in the Sales > Orders > Creditmemos under view memo section.

Cart Product Comment query is used to add comments to the shopping cart and show comments. You can also show comments in order, invoice, shipping and credit memo.
You can set Cart Product Comment in GraphQL/REST API with default Magento query.

If you want to display cart comment then need to enable Cart Product Comment extension.

The following query to check Cart Comment extension status.

{
    storeConfig {
      dolphin_cart_comment_status
    }
}

Use comment parameter in default Magento query for add and show the comment.

mutation {
    updateCartItems(
      input: {
        cart_id: "HeClKlav1RvxnDaC9GkPDDksEbPJDRxB",
        cart_items: [
          {
            cart_item_id: 35
            quantity: 2
            comment: "test 35"
          }
        ]
      }
    ){
      cart {
        items {
          id
          quantity
          comment
          product {
            id
            name
            sku
          }
          quantity
        }
      }
    }
}

{
    cart(cart_id: "HeClKlav1RvxnDaC9GkPDDksEbPJDRxB") {
      items {
        id
        quantity
        comment
        product {
          name
          sku
        }
      }
    }
}

{
    customer {
      orders(filter: {number: {eq: "000000007"}}) {
        items {
          id
          increment_id
          order_date
          status
          items {
            id
            quantity_ordered
            ... on OrderItem {
              comment
            }
            ... on DownloadableOrderItem {
              comment
            }
            ... on BundleOrderItem {
              comment
            }
          }
          invoices {
            id
            items {
              id
              quantity_invoiced
              ... on InvoiceItemInterface {
                order_item {
                  ... on OrderItem {
                    comment
                  }
                  ... on DownloadableOrderItem {
                    comment
                  }
                  ... on BundleOrderItem {
                    comment
                  }
                }
              }
            }
          }
          shipments {
            id
            items {
              id
              quantity_shipped
              ... on ShipmentItemInterface {
                order_item {
                  comment
                  product_name
                  product_sku
                }
              }
            }
          }
          credit_memos {
            id
            items {
              id
              product_name
              product_sku
              quantity_refunded
              ... on CreditMemoItem {
                order_item {
                  ... on OrderItem {
                    comment
                  }
                }
              }
              ... on BundleCreditMemoItem {
                order_item {
                  comment
                }
              }
              ... on DownloadableCreditMemoItem {
                order_item {
                  comment
                }
              }
            }
          }
        }
      }
    }
}

Please check the following example for Cart Product Comment.

GET - yourdomain.com/rest/V1/orders/7/

POST - yourdomain.com/rest/V1/carts/mine/items

{
  "cartItem": {
    "sku": "24-MB03",
    "qty": 1,
    "quote_id": "38",
    "extension_attributes": {
        "comments": "rest comment"
    }
  }
}

{
    "item_id": 62,
    "sku": "24-MB03",
    "qty": 5,
    "name": "Crown Summit Backpack",
    "price": 38,
    "product_type": "simple",
    "quote_id": "36",
    "extension_attributes": {
        "comments": "rest comment"
    }
}