Alias is a dedicated authentication provider that allows you to authenticate your users, and contact them without using email. See the Alias website for more on what Alias can do, and why you should use it for your service.
Alias implements the OAuth 2.0 and OpenID Connect standards. It's recommended that you familiarise yourself with these standards. The oauth.com guide is a good place to start - in particular, the section on OpenID Connect.
The quickstart guide will help you configure an OpenID Connect integration to use Alias as an auth provider.
Once your developer access has been granted, you'd be able to create API clients and get on with working with Alias.
From time to time, both the OAuth 2.0 provider and REST API may have undocumented features. However, only features explicitly included in this documentation are supported and maintained. Undocumented features may be changed or removed at any point, without warning.
It is highly recommended that you do not rely on any undocumented features.