We have saas based project in which all client connect their webshop on my system. Once connected we will do below thing using API
How can I authenticate without app? If App is require then what type of APP I have to create.
You will need to create an app in order for merchants to install it and utilize the integration. If this is for a single merchant you can use a Custom App, otherwise, you can create a public app. This gives you access to OAuth: https://shopify.dev/tutorials/authenticate-with-oauth
You will need to connect to the Shopify API in order to edit the product in that way.