GMail Mailboxes vs Labels
GMail technically doesn’t use mailboxes or folders. Instead it uses “labels”. Labels appear as folders (i.e. mailboxes) in email clients such as Outlook or Thunderbird, or as mailboxes in the IMAP protocol.
I recommend comparing how your GMail looks in Outlook (or Thunderbird) vs the web browser at Gmail.com.
Notice how an email can be given multiple labels, and it will appear simultaneously in each of the “mailboxes” in Outlook.