This is maybe sounds dumb, but the thing is i already have the backend already hosted with all functionality to assert my client needs. Is it possible to just the frontend part of shopify but all the data come from external. It cover auth, product, order, cart, etc
Custom storefronts
https://shopify.dev/custom-storefronts
Or more simply buy buttons to embed shopify
https://help.shopify.com/en/manual/online-sales-channels/buy-button