WordPress has three types of plugins: regular, must-use, and drop-in, which enhance site functionality. Regular plugins are easily managed via the Admin Area and can be activated with clicks. Must-use plugins are for advanced users and auto-enable without being listed in regular plugins. Drop-in plugins modify core features by placing specific PHP files in the WP directory and are also not listed among regular plugins. Additionally, the theme's functions.php file allows customization and functionality additions tied to that specific theme. Regular plugins are generally preferred for broader customization, while must-use and drop-in plugins serve specific roles. Theme functions are ideal for theme-specific modifications.
https://digwp.com/2025/01/wordpress-plugins-must-use-drop-in/