In order to log in using SSO, you need your users to exist in the Atlassian product. There are several ways to achieve this and you can read more about the different user provisioning options if you are unsure of which one to choose.
2. SSO protocol
You also need to select an SSO protocol. The protocols authenticate users, provide identity data for access control and function as a communication method for a user’s identity. SAML and OpenID Connect (OIDC) are the most widely used identity protocols for web-based SSO. Read about the difference between SAML and OIDC (OpenID Connect) if you want to know more.