How to get the current source within field process

I were able to solve this issue with a custom source plugin:
mymodule/src/Plugin/migrate/source/UrlWithActiveSourceUrl.php

<?php
namespace Drupal\mymodule\Plugin\migrate\source;

use Drupal\migrate_plus\Plugin\migrate\source\Url;
use Drupal\migrate\Row;

/**
* Source plugin for retrieving data via URLs and add active URL as variable.
*
* @MigrateSource(
*   id = "url_with_active_source_url"
* )
*/

class UrlWithActiveSourceUrl extends Url  {

  /**
  * {@inheritdoc}
  */
  public function prepareRow(Row $row) {
    $migration = $this->migration->getSourcePlugin();
    $dataParser = $migration->dataParserPlugin;
    $activeUrl = $dataParser->currentUrl();

    $row->setSourceProperty('active_url', $activeUrl);
    return parent::prepareRow($row);
  }
}

Now I can use in my migration config:

source:
  plugin: url_with_exhibit_url
  data_fetcher_plugin: http
  data_parser_plugin: json
  track_changes: true
  urls:
    - https://www.url1.de/data.json
    - https://www.url2.de/data.json
    - https://www.url3.de/data.json

And now within process, I can use "active_url" whereever needed (e.g.)

process:
  _active_source_url: active_url

I found one problem with my implementation: if there is a source change, the last item BEFORE the source change already uses the new source-URL. Seems that $dataParser->currentUrl() does not return the correct item ;-(
Any idea? I can't access the property $dataParser->activeUrl or $dataParser->urls - as those are protected properties ;-(

As it looks we definitely have a problem using the $dataParser->currentUrl() inside a source plugin. As a solution for myself I now use a data_parser plugin - following @hctom's comment ( https://www.drupal.org/project/migrate_plus/issues/3050274#comment-13935165 → ):
1. Create a file within your custom module (mymodule/src/Plugin/migrate_plus/data_parser/CustomJson.php)
2. Add to CustomJson.php

<?php

namespace Drupal\mymodule\Plugin\migrate_plus\data_parser;

use Drupal\migrate_plus\Plugin\migrate_plus\data_parser\Json;

/**
 * Obtain JSON data for migration.
 *
 * @DataParser(
 *   id = "custom_json",
 *   title = @Translation("JSON Parser for My Module")
 * )
 */
class CustomJson extends Json {
  
  /**
   * {@inheritdoc}
   */
  protected function fetchNextRow(): void {
    parent::fetchNextRow();

    // Inject file metadata.
    if ($this->valid()) {
      $activeUrl = $this->currentUrl();
      $activeUrlParts = explode('/api/', $activeUrl);
      $activeBaseUrl = str_replace('https://', 'https://username:userpassword@', $activeUrlParts[0]);

      $this->currentItem['remote_auth_url'] = $activeBaseUrl;
      $this->currentItem['active_source_url'] = $activeUrl;
    }
  }
}

Now within your migration use:

source:
  plugin: url
  data_fetcher_plugin: http
  data_parser_plugin: omeka_json

Within the process section you will now be able to use "active_source_url" and other defined parameters.

process:
  _active_source_url: active_source_url