Responsive rules with block themes and theme.json

They’re defined the same way they always have been, in a CSS file.

Do what styling you can using the block editor and theme.json then supplement it with additional CSS in a CSS file. You can enqueue it the same was as a classic theme in functions.php, and if it’s a minor change there’s a custom CSS option in the site editors global styles section.

Block styles and block variations may help here by providing useful point and click UIs that automatically add the HTML classes you need so the user doesn’t have to type them out themselves. You might also find that using the built in layout functions might interfere, e.g. if you were defining flexbox layouts and grid layouts for a group block it would not make sense to turn it into a grid/row/stack.

Enqueue your CSS file the same way you would have done in a classic theme, just be sure to also add it as an editor stylesheet so that the site editor displays things correctly.