html/bootstrap menu to wordpress with dropdown

Try this walker.

Example usage:

wp_nav_menu( [
    'menu_class'     => 'menu-horizontal text-left',
    'container'      => '',
    'depth'          => 2,
    'theme_location' => 'main-menu',
    // You should have loaded the class-my-bs-walker-nav-menu.php file from
    // within the theme's functions.php file.
    'walker'         => new My_BS_Walker_Nav_Menu,
] );

The walker is not intended as a “global” walker for any Bootstrap menus; instead, it was intended just for the menu structure that you provided in your question.