How to migrate node/user/etc aliases

discussion_aliases.yml

id: discussion_aliases
label: Discussion aliases
source:
  plugin: d7_node
  node_type: discussion
  constants:
    english: en
    node: /node
    empty: ""
process:
  alias:
    plugin: concat
    delimiter: /
    source:
      - constants/empty
      - path
  langcode: constants/english
  destination_nid:
    -
       plugin: migration
       migration: community_discussions
       source: nid
       no_stub: true
    -
       plugin: skip_on_empty
       method: row
  source:
    plugin: concat
    delimiter: /
    source:
      - constants/node
      - @destination_nid
destination:
  plugin: url_alias

sc_migrate.module

<?php

use Drupal\migrate\Plugin\MigrateSourceInterface;
use Drupal\migrate\Plugin\MigrationInterface;
use Drupal\migrate\Row;

/**
 * Implements hook_migrate_prepare_row().
 */
function sc_migrate_migrate_prepare_row(Row $row, MigrateSourceInterface $source, MigrationInterface $migration) {
  $args = [
    ':language_none' => 'und',
  ];
  switch ($migration->id()) {
    case 'discussion_aliases':
    case 'announcement_aliases':
      $args[':system'] = 'node/' . $row->getSourceProperty('nid');
      break;
    case 'community_user_aliases':
      $args[':system'] = 'user/' . $row->getSourceProperty('uid');
      break;
    default:
      return;
  }
  /** @var Drupal\migrate\Plugin\migrate\source\SqlBase $source */
  $row->setSourceProperty('path', $source->getDatabase()
    ->query('SELECT alias FROM {url_alias} WHERE source IN (:system) AND language = :language_none ORDER BY pid ASC', $args)
    ->fetchField());
}