How do I make a link to a flagging edit page?
Digging into the flag module a little, I can see this entry in flag.routing.yml that appears to be for your Flagging edit screen.
flag.field_entry.edit:
  path: '/flag/details/edit/{flag}/{entity_id}'
  defaults:
    _controller: '\Drupal\flag\Controller\FieldEntryFormController::edit'
    _title_callback: '\Drupal\flag\Controller\FieldEntryFormController::editTitle'
  requirements:
    _flag_access: 'entity:flag{flag}'

You can build out the link to the flagging manually, e.g.
&lt;a href='/flag/details/edit/FLAG_ID/FLAGGING_ID'&gt;Edit My Flagging&lt;/a&gt;

Or if you're working with twig, e.g. rewriting a view's field, make sure you're also including the flag's id and flagging's id in the view then rewrite doing something like:
&lt;a href='/flag/details/edit/{{ FLAG_ID }}/{{ FLAGGING_ID }}'&gt;{{ &quot;Edit Flagging&quot;|t }}&lt;/a&gt;

Or you can use PHP to build the url directly from the route. It's recommended to use one of the following methods if you're working in PHP.
To get just the url text or object in PHP, use Url::fromRoute():
use \Drupal\Core\Url;

$my_flagging_url_object = Url::fromRoute('flag.field_entry.edit', [
  'flag' =&gt; FLAG_ID,
  'entity_id' =&gt; FLAGGING_ID,
]);
// $my_flagging_url_string returns something like &quot;/flag/details/edit/123/1234&quot;
$my_flagging_url_string = $my_flagging_url_object-&gt;toString();

To build the whole link in PHP, use Link::createFromRoute():
use \Drupal\core\Link;

$my_flagging_edit_link = Link::createFromRoute('MY FLAGGING EDIT TEXT', 'flag.field_entry.edit', [
  'flag' =&gt; FLAG_ID,
  'entity_id' =&gt; FLAGGING_ID,
]);