Hi, happy to help here.
On Windows, open Outlook, go to the View tab, select Change View to pick Compact, Single, or Preview, and adjust Layout settings for Folder Pane, Reading Pane, and To-Do Bar. On Outlook web, full classic view is no longer available, but you can adjust message list and reading pane under Settings > View all Outlook settings > Layout.
Are you aiming for the full classic interface or just the old message layout?