buddyboss-modifications.php

add_action( 'bp_settings_setup_nav', 'bbm_add_change_passcode_nav_items' );

function bbm_add_change_passcode_nav_items() {
	bp_core_new_subnav_item( array(
		'name'            => __( 'Billing', 'buddyboss' ),
		'slug'            => 'billing',
		'parent_url'      => trailingslashit( get_site_url() ),
		'parent_slug'     => 'settings',
		'screen_function' => 'bp_settings_screen_billing',
		'position'        => 25,
		'user_has_access' => true,
	), 'members' );
	bp_core_new_subnav_item( array(
		'name'            => __( 'Change Passcode', 'buddyboss' ),
		'slug'            => 'change-passcode',
		'parent_url'      => trailingslashit( get_site_url() ),
		'parent_slug'     => 'settings',
		'screen_function' => 'bp_settings_screen_changepasscode',
		'position'        => 26,
		'user_has_access' => true,
	), 'members' );
}

The URL slug they go to is set by 'parent_url' and 'slug' in the options array. The slug is just appended to the parent URL when being displayed. In the billing button example, that would be "https://example.com/billing", where example.com would be the site URL.
The screen_function doesn't have a purpose in this case, I believe it might be used if you wanted to display a page inside the account management area. I haven't tested that though.

The menu icon is set via CSS, you just need to set the "content" value.  The HTML element ID looks like it's being set from the slug value, so in the example above I would use the following CSS to set the icon to a shopping cart:

#buddypress .bp-esttings-container .bp-navs a#billing:before {  
  content: '\E905';
}

All of the icons can be found on this page: https://www.buddyboss.com/resources/font-cheatsheet/