It’s unlikely that we’ll see this experimental version merged into core with WordPress 5.6. According to Carolina Nymark,

The theme will need Gutenberg and the Full Site Editing experiment to be enabled. It will not be part of Core, but once complete it will be available in the theme directory.

At the time of this writing, Twenty Twenty-One Blocks requires the Gutenberg plugin. Once installed and activated both the theme and the plugin, a Site Editor menu item appears in your WordPress admin menu (you don’t need to activate the Full Site Editing experiment anymore).

Now, click on the new Site Editor menu item to open the full-site editing interface and start editing any element on the page using the block editor.