Where is hostname?
You might be looking for hostname in App+Web. The concept of hostname has been replaced by data streams because you can have both app and web-based data streaming into your Google Analytics property.
You configure your streams in the Data Stream section of settings. More about that here.
Stream name as a reportable dimension, however, does not match what you named your stream in the settings. Stream name defaults to the type of stream (web or app).
To see a unique value that represents the hostname, you will need to use Stream ID for now.
You can see Stream ID next to page path in the Exploration report in the Analysis Hub.
Obviously, this is not an ideal solution so we are hoping this will be updated as App+Web continues to be developed.