GitHub Single Sign On to IDP, is not true Single Sign on, that you may see in other Solutions.
You still use a GitHub account and login to do all your GitHub activity, it just provides additional linkage and authentication to a configured company IdP.
When you access resources within a GitHub organization that uses SAML SSO, GitHub will redirect you to the organization’s SAML IdP to authenticate. After you successfully authenticate with your account on the IdP, the IdP redirects you back to GitHub, where you can access the organization’s resources.
If you have recently authenticated with your organization’s SAML IdP in your browser, you are automatically authorized when you access a GitHub organization that uses SAML SSO. If you haven’t recently authenticated with your organization’s SAML IdP in your browser, you must authenticate at the SAML IdP before you can access the organization.
You must periodically authenticate with your SAML IdP to authenticate and gain access to the organization’s resources on GitHub.com. The duration of this login period is specified by your IdP and is generally 24 hours. This periodic login requirement limits the length of access and requires you to re-identify yourself to continue.