All default authentication pages (login, logout, forgotten password, etc.) are automatically attempting to load custom.js file that can be used for manipulation with page UI. This file is common to all those pages and the expected location is application-root-folder/assets/identity/js/custom.js.
Among other things you can hide or modify all standard buttons on the authentication pages (Login, Cancel, Submit…) referencing their ids.
External Sign-in Buttons
If the application is using external authentication providers, the login page contains for each provider a dedicated login button. The id of such button is in form loginButton-AuthenticationScheme , where AuthenticationScheme is Google , Microsoft or AzureAd .