Are User ID and Visitor ID linked ?

[#1]

1. can a user have a visitor ID?

2. can a visitor have a User ID?

3. we have an app that has pre and post logins - is pre- login "Visitor", and post-login "User"?

[#2]

In short, the answer is yes.
Consider reviewing the following documentations:
https://docs.adobe.com/content/help/en/analytics/technotes/visitor-identification.html

https://docs.adobe.com/content/help/en/id-service/using/intro/overview.html

https://docs.adobe.com/content/help/en/id-service/using/id-service-api/methods/setcustomerids.html