Using New Outlook on Windows for personal email, calendar, and contact management
There is not a separate Outlook version you can download to get Unified Inbox.
Microsoft announced the new All Accounts / Unified Inbox for new Outlook for Windows under Roadmap ID 564609, with August 2026 listed as the planned rollout timeframe. That date was a rollout target, not a guarantee that every user would receive it in August. Microsoft also states that roadmap dates are estimates and can change. The current status is still "in development," though. See this link https://www.microsoft.com/en-us/microsoft-365/roadmap?filters=&searchterms=564609
You can continue to use the link above to monitor its status.