In this video, we go over what a Business to Business (B2) external Azure AD account is and how to enforce Multi-Factor Authentication (MFA) on guest accounts with a Condition Access Policy. The demo goes over disabling SMS authentication, then inviting guest users to the tenant. After that, we walk through enabling MFA with the Microsoft Authenticator app.
Zero to Hero: https://www.ciraltos.com/zero-to-hero-with-windows-virtual-desktop/
Monthly Active User Pricing: https://docs.microsoft.com/en-us/azure/active-directory/external-identities/external-identities-pricing