Authenticate the sites in SharePoint online with Azure AD

I want 100,000 external users to have access to my SharePoint online Site collection.

I was thinking of going the Azure AD route, where external users will have there ID's created in Azure AD cloud.

Trying to figure how I can integrate Azure AD cloud with my SharePoint Online Site collection.

Currently my site collection is tied to On-premise AD.

Is there a way to integrate the SharePoint online to use both Azure AD and On-premise AD?

Thanks

Nate