How to add custom php file to right sidebar?

There are three ways to do that:

1. Create a widget (Recommended)
   Here is official documentation. You can include your php file within widget() function of your widget class.

2. Create and place a shortcode in exiting wordpress default text widget.
   Here is sample code

   //Place this code in theme's functions.php
   
   add_shortcode('ContactForm', 'cf_shortcode');
   
   function cf_shortcode($att) {
      ob_start();
      include "path/to/php/file";
      return ob_get_clean();
   }
   

Then place [ContactForm] in wordpress default text widget.

Create a child theme and override sidebar.php file of your theme there.