I'm building an application in which my clients can provide the location ID of their business/shop from Google. On their behalf, I want to download all opinions (not just 5, but all) for a given place and display them in my application. Is it possible?
You have to create an OAuth 2.0 client ID if you want to use the GBP API anyways: https://developers.google.com/my-business/content/basic-setup#request-client-id
You will then have to integrate the authorization flow into your frontend so the end user can grant the application access to their locations.