This is generally done by using the domain configuration. For example, all users with an email address domain of acme.com can be configured to use a particular SAML or OpenID Connect configuration.

As soon as you configure one IdP with a domain, the login panel will collect the email address first to understand if we need to ask for a password or forward them along to a federated identity provider.

Read more about managed domains here: https://fusionauth.io/docs/v1/tech/identity-providers/openid-connect/