$wpdb is a PHP global variable that holds the WordPress database object, which is actually an instantiation of the wpdb class.
WP_Query actually uses wpdb to query the database.
Use WP_Query when you manipulate native WordPress tables (the vast majority of common tasks).
Use $wpdb when you manipulate custom tables.
References
- https://developer.wordpress.org/reference/classes/wpdb
- https://developer.wordpress.org/reference/classes/wp_query/
- Data Validation (must read)
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