Concatenate output of php function into string

You can use output buffering like:

$output_settings .= '';

ob_start();
settings_fields('fahad_plugin_cpt_settings');
submit_button('Delete', 'btn btn-danger btn-sm', 'submit', false);
$output_settings .= ob_get_clean();

and use $output_settings variable in your code like

// ...your code...
$data .= '<form method="post" action="options.php">';
$data .= $output_settings;
$data .= '</form>';
// ...your code...