Fetch Guest Customer Quote data by Masked Id /V1/guest-carts/:cartId REST API Magento 2.

You can Fetch Guest customer quote data using REST API in Magento 2 by passing cart Masked ID as parameter in Request URL,

/V1/guest-carts/:cartId

Action: GET

URL: <WEBSITE_URL>/rest/<STORE_CODE>/V1/guest-carts/:cartId  (Example: https://magento.test/rest/default/V1/guest-carts/fQtl1bxLdxud1fFEP68znOAPnAFFpjVh)

Payload: Empty

Response:

{
    "id": 5,
    "created_at": "2022-07-13 13:13:35",
    "updated_at": "2022-07-13 13:13:35",
    "is_active": true,
    "is_virtual": false,
    "items": [],
    "items_count": 0,
    "items_qty": 0,
    "customer": {
        "email": null,
        "firstname": null,
        "lastname": null
    },
    "billing_address": {
        "id": 10,
        "region": null,
        "region_id": null,
        "region_code": null,
        "country_id": null,
        "street": [
            ""
        ],
        "telephone": null,
        "postcode": null,
        "city": null,
        "firstname": null,
        "lastname": null,
        "email": null,
        "same_as_billing": 0,
        "save_in_address_book": 0
    },
    "orig_order_id": 0,
    "currency": {
        "global_currency_code": "EUR",
        "base_currency_code": "EUR",
        "store_currency_code": "EUR",
        "quote_currency_code": "EUR",
        "store_to_base_rate": 0,
        "store_to_quote_rate": 0,
        "base_to_global_rate": 1,
        "base_to_quote_rate": 1
    },
    "customer_is_guest": true,
    "customer_note_notify": true,
    "customer_tax_class_id": 3,
    "store_id": 2,
    "extension_attributes": {
        "shipping_assignments": []
    }
}

The Response of the Guest Customer Quote will be guest details from the quote table.

You may also be interested in the Flow of Guest Customer Place Order by REST API Magento 2.