Setup ADFS For External Access

Hi all, I would like to setup ADFS for the following scenario below:

Internal intranet:

URL: https://intranet.acme.com

domain: ACME

ACME domain users: Acme\johndoe

External Vendor:

domain: ABC

ABC domain users: ABC\lucysmith

Goals: allow external vendor users ABC domain access internal intranet https://intranet.acme.com via SharePoint-ADFS

Questions:

1. Do I need to setup ADFS on both ACME & ABC domain or just one side? If it is one side, then which one - ACME or ABC?

2. When I setup SharePoint web application for https://intranet.acme.com, will this URL will be served for both internal and external users or do I have to extend it as different URL for external users?

         a. If https://intranet.acme.com served for both internal and external vendor users, will internal user get normal NT prompt for authentication or it will redirect to ADFS login page just like external user?

         b. If we need to extend web application for external vendor user, let's sayhttps://abcexternal.acme.com, will we only need to config adfs for this extended web application so external vendor user will get adfs redirect login where internal user got NT prompt for authentication?

Thanks