Where are the layout defaults in Twenty Twenty Two coming from?

there’s a layout option for the post content block that says it will inherit the default layout. Where is that default coming from?

In block themes, the default layout settings (contentSize and wideSize) can be found in the theme.json file in the root theme folder, e.g. at wp-content/themes/twentytwentytwo/theme.json in Twenty Twenty-Two. _{See source on GitHub for WordPress v5.9.3 & Twenty Twenty-Two v1.1}

And if the layout option does not exist in the theme.json file, i.e. settings.layout is not defined, then the “Layout” panel (in the Block Settings sidebar) will not include the toggle labeled “Inherit default layout”.