How to login to SharePoint?

It depends on its version (on-prem or online) and deployment. Nowadays majority of users login to SharePoint via Office 365. For SharePoint Online, users typically access through portal.office.com or directly via [tenant].sharepoint.com URLs. Authentication methods include: single sign-on (SSO) with Active Directory, multi-factor authentication (MFA), federated authentication, and modern authentication protocols. On-premises SharePoint may use Windows authentication, forms-based authentication, or SAML-based authentication depending on configuration.