How can I maintain redirects from a non-existing path to an existing one?
After you've installed and enabled the "Redirect" module (https://www.drupal.org/project/redirect), you can manage your redirects by going to the redirect list page at /admin/config/search/redirect:



You can find this route and all the other routes provided by the "Redirect" module in redirect.routing.yml:

redirect.list:
  path: '/admin/config/search/redirect'
  defaults:
    _entity_list: 'redirect'
    _title: 'URL redirects'
  requirements:
    _permission: 'administer redirects'