where add plugin stylesheet in wordpress code example

Example 1: enqueue wordpress

function wpdocs_theme_name_scripts() {
    wp_enqueue_style( 'style-name', get_stylesheet_uri() ); /* enqueues style.css */
    /* if you want to enqueue other styles use: */
    /* wp_enqueue_style( 'style-name', get_template_directory_uri() . '/css/your-style-name.css' ); */
    wp_enqueue_script( 'script-name', get_template_directory_uri() . '/js/example.js', array(), '1.0.0', true );
}
add_action( 'wp_enqueue_scripts', 'wpdocs_theme_name_scripts' );

Example 2: how to add css file in wordpress

wp_enqueue_style( 'style', get_stylesheet_uri() );