Since the visitors are unlimited, do they have to be added to the sheet with their email and name to use the app as a visitor?
No. Visitors do not sign in to your App. As soon as a visitor signs in, they become a User.
When visitors open the app, no record is ever created in the users table. Does this also mean that the app cannot make use of features like roles, row-owners and user-specific columns which are dependent on a user profile record?
Roles & Row Owners require User Profiles. User Specific columns do not.
If a visitor adds data to a user-specific column, then exits the app and returns later, will Glide retrieve the visitor entered data in the user-specific column? How does Glide know which data belongs to which visitor without a user profile?
The data won’t be retained across sessions forr visitors.
No.
It doesn’t. As Thinh noted, user specific data won’t persist across sessions for visitors. But it can still be used within a single session.