As a web application, is there a way to connect to a customer Shopify store without having to create an app on Shopify ?
I would like to get access to specific data (once the customer grant me access) and use it for my web application.
A bit like this Facebook example: (but basically where my web application will connect to Shopify by requesting access to specific data to the customer).
Facebook's Oauth flow actually looks pretty similar to ours. Once the user clicks an install link, they're brought to a page on Shopify to confirm scopes, and then they're redirected back to the app afterwards. However this process does involve creating an app through your Partner dashboard. The dashboard allows you to get the necessary credentials to interact with the API, but the scopes you request are coded into the app. You just need to provide an install link to the merchant, and once they click it they will enter the Oauth flow. Our full Oauth docs are here.
If you don't want to go that route, you can also use a private app. This requires the merchant to create API credentials from a page in their admin, and also choose the access scopes. Once you have the credentials, you can make calls from your application.