We've built an open-source React Checkout that works with any Magento 2 shop.
It's available for free at https://github.com/hyva-themes/hyva-checkout/
About / Hyvä Checkout
Hyvä Checkout uses only the cart-token and customer-token to communicate with Magento 2 through GraphQL.
With Hyvä Themes and default Magento, these tokens are stored in customerData that are in LocalStorage.
PWA Studio uses the same tokens in localStorage, but under a different key.