Build 21390 focused heavily on refining the UI and testing the backend servicing infrastructure.

Furthermore, between June 7 and June 14, 2021, Microsoft used Build 21390 to test its "servicing pipeline" by releasing multiple cumulative updates (21390.1000, 21390.1010, and 21390.2025) on top of the same base build. This was essential for ensuring that the transition to future versions would be smooth.

This build introduced new, modern icons for the Task Manager and MSI installers . These icons swapped out older skeuomorphic designs for the sleeker Fluent Design language that was a precursor to Windows 11.

Users could set Windows Terminal Preview (version 1.9 or higher) as the default terminal emulator for the first time. This allowed Command Prompt and PowerShell to open directly within the modern Terminal interface.

Several fixes addressed visibility issues, specifically making text readable in the File Explorer search box when using dark theme. Why This Build Is Unique