Cover both eras: "I register sidebars with register_sidebar, display them with dynamic_sidebar. Since WordPress 5.8, widget areas accept blocks — so users can put paragraphs, images, or any block in the sidebar, not just legacy widgets."
Sidebars are widget-ready areas registered with register_sidebar() in functions.php, hooked to widgets_init. They define before_widget, after_widget, before_title, and after_title HTML wrappers. Display them in templates with dynamic_sidebar(). Since WordPress 5.8, the widget editor uses blocks — users can add any block to widget areas, not just traditional widgets. Classic widgets still work alongside blocks. Strong candidates explain: the difference between sidebars (the area) and widgets (the content placed in them), that a theme can have multiple sidebars (header, footer, sidebar), and that the block-based editor provides much more flexibility but some themes still use the classic widget screen via the Classic Widgets plugin.
Tests knowledge of WordPress widget system. Candidates who only know the classic widget approach are behind on the block-based transition. Those who understand both and can explain the migration path demonstrate current WordPress knowledge.