Migrate Multiple URLs from a Spreadsheet into ONE Link Field
The two main ways to approach this with php are via hook or plugin.
The first is with a hook, specifically, hook_migrate_MIGRATION_ID_prepare_row().

Allows adding data to a row for a migration with the specified ID.

For example, you could combine your two link columns to a new column formatted as an array.
function my_module_migrate_MY_MIGRATION_ID_prepare_row(Row $row, MigrateSourceInterface $source, MigrationInterface $migration) {
  // Assuming your url fields are keyed event_url and ticketing_url in your migration.yml.
  $links_array = [];
  if ($row-&gt;hasSourceProperty('event_url')) {
    $event_url = $row-&gt;getSourceProperty('event_url');
    if (!empty($event_url)) {
      $links_array[] = [
        'title' =&gt; 'Event URL',
        'uri' =&gt; $event_url,
      ];
    }
  }
  if ($row-&gt;hasSourceProperty('ticketing_url')) {
    $ticketing_url = $row-&gt;getSourceProperty('ticketing_url');
    if (!empty($ticketing_url)) {
      $links_array[] = [
        'title' =&gt; 'Ticketing URL',
        'uri' =&gt; $ticketing_url,
      ];
    }
  }

  $row-&gt;setSourceProperty('event_and_ticketing_url', $links_array);
}

You can use your programmatically added source value in your process section like:
process:
  ...
  field_links: event_and_ticketing_url
  ...


I prefer to use plugins over hooks whenever possible for a few reasons, but one is because plugins are more visible to anybody looking at the implementation later. By looking at the migration.yml, a future developer will know that a custom plugin does something to this thing and the plugin can be found in the src/Plugin directory.
An example doing the same thing, but with a plugin might look like:
my_module/src/Plugins/migrate/process/EventAndTicketingLinks.php:
/**
 * Process plugin to combine event and ticketing url sources to one links array.
 *
 * Example Usage:
 *
 * field_my_links_field:
 *   plugin: my_module_event_and_ticketing_links
 *   link_fields:
 *     event: event_url
 *     ticketing: ticketing_url
 *
 * @MigrateProcessPlugin(
 *   id = &quot;my_module_event_and_ticketing_links&quot;,
 * )
 */
class EventAndTicketingLinks extends ProcessPluginBase {

  /**
   * {@inheritdoc}
   */
  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
    $config = $this-&gt;configuration;
    $links = [];
    
    if (!empty($config['link_fields']['event'])) {
      $event_url = $row-&gt;getSourceProperty($config['link_fields']['event']);
      if (!empty($event_url)) {
        $links[] = [
          'title' =&gt; 'Event Url',
          'uri' =&gt; $event_url,
        ];
      }
    }
    if (!empty($config['link_fields']['ticketing'])) {
      $ticketing_url = $row-&gt;getSourceProperty($config['link_fields']['ticketing']);
      if (!empty($ticketing_url)) {
        $links[] = [
          'title' =&gt; 'Ticketing Url',
          'uri' =&gt; $ticketing_url,
        ];
      }
    }
    
    return $links;
  }
  
}


Additionally, since you ended up combining the urls into one source column anyway, you could use the hook or a plugin to work with that data too.