Inbox opens the first mailbox you can use. A mailbox list appears only when there is more than one, and an open message replaces the folder list, with the folder in the page bar to return to it. Until there is a mailbox, Inbox shows the next setup step: add a domain, verify it, or add a mailbox.
On Domains, Add domain asks only for the domain name and then opens the domain's page. That page has the verification record, the Unknown addresses setting, which now saves when it is chosen, and the DNS records with the status of each.
On Addresses, Add address adds a mailbox, alias, or reject address from one drawer. Mailbox storage, suspension, and removal moved there from Inbox.