Yes, wp_nav_menu displays all levels of menu by default.
You don’t have to write your custom walker class to display submenu items – WordPress already has one which is used by default.
WP_Bootstrap_Navwalker is used only because given theme uses Bootstrap and nav menu should generate custom HTML code, which is compatible with Bootstrap.
The only thing you have to remember is that there is depth argument which decides how many levels of menu should be printed.