OAuth has two scenarios in real mode. The normal and default style of access is called online. In some cases, your application may need to access a Google API when the user is not present,It's offline scenarios . a refresh token is obtained in offline scenarios during the first authorization code exchange.
So you can get refersh_token is some scenarios ,not all.
you can have the content in https://developers.google.com/identity/protocols/OAuth2WebServer#offline .