Adobe Assets and Content-Hub

1.Content hub is usually included as part of Asset Ultimate license .

2. you can share your assets/folder to external anonymous users using share link functionality of AEM Assets .  Users can hit the link and download/view asset without logging in. There is no view of metadata though. CH also has share link but that only works for user having login access on CH.

1.Is Content Hub included as part of Adobe Assets, or does it require a separate license?

Assets Cloud Services SKU (non Ultimate) - get 250 CH users already included. This SKU does not have ability to purchase beyond 250 CH users. For that you will need to purchase Assets Ultimate, which gets 250 CH users included, and additional can be purchased (your account exec can share details on pricing). 

2.Can assets be shared externally with users outside our firewall? Currently, we use Brandfolder, which allows external users to download assets. Is a similar external sharing experience possible with Content Hub or Adobe Assets?

3.Are there any recommended workarounds or solutions within Adobe Assets to enable external asset sharing possibly with UI where assets are organized and can be downloaded?

digarg answered it correctly, Content Hub has Public Link Share capability (link valid for max 7 days only)