Design Team says
Twenty Twenty-One will use a modified version of the Seedlet theme as its base. This provides us with a thorough system of nested CSS variables to make child theming easier, and to help integrate with the global styles functionality that’s under development for full-site editing.
Once the theme is stable, after Beta 1, we’ll start exploring Full Site Editing support.
To be honest, Twenty Twenty-One is like other WordPress default theme. Minimal and simple