M365 (Microsoft 365) is the cloud productivity suite. Most tickets: 'can't log in', 'lost MFA device', 'shared mailbox missing', 'OneDrive not syncing'.
Admin Center (admin.microsoft.com) is where you reset passwords, restore deleted users (30-day window), and assign licenses. Entra ID (formerly Azure AD (Active Directory)) is the underlying identity layer.
When MFA breaks, you reset the user's authentication methods — never disable MFA company-wide.