The start URL should only be the bit you have blocked out in red, what you may consider the "home page" or "base page" of your website. For example, Google's 'start_url' would be https://google.com.
This is the URL a user would come to when they first start your PWA, or have finished a line of visit and want to go back to the start of the site.
See here for more info: https://developer.mozilla.org/en-US/docs/Web/Manifest/start_url