I know this questions is already answered. But it is missing some main points those are my findings i am adding those points. The source of this answer is here.
1- Azure AD External Identities was previous name of Azure AD B2C.
Azure B2c is a business to consumer identity management system.
2- Microsoft Entra External ID
Microsoft Entra External ID is a combination of Azure Ad B2C and Ad B2B (Now Entra Id).
When you create tenant of Microsoft Entra External ID you system create two type of tenants.
Workforce (B2B)
External (B2C)
WorkForce tenant is used for Ad B2B (Entra Id) Operation.
External Tenant is used for Azure B2c Operations.
To Learn more about Microsoft Entra External ID check here