Examples 

Relation to pages and content elements 

A conference refers to pages and content elements with more information:

The related pages and content of a conference

The related pages and content of a conference

EXT:my_extension/Configuration/TCA/Overrides/157-tx_myextension_conference-related_content.php 
<?php

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;

defined('TYPO3') or die();

ExtensionManagementUtility::addTCAcolumns('tx_myextension_conference', [
  'related_content' => [
    'label' => 'my_extension.db:conference.related_content',
    'config' => [
      'type' => 'group',
      'allowed' => 'pages,tt_content',
      'elementBrowserEntryPoints' => [
        '_default' => '###SITEROOT###',
      ],
      'suggestOptions' => [
        'default' => [
          'searchWholePhrase' => true,
        ],
        // Only standard pages
        'pages' => [
          'searchCondition' => 'doktype = 1',
        ],
        'tt_content' => [
          'additionalSearchFields' => 'bodytext',
        ],
      ],
    ],
  ],
]);
ExtensionManagementUtility::addToAllTCAtypes(
  'tx_myextension_conference',
  'related_content',
  '',
  'before:--div--;core.form.tabs:categories',
);
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Relation to the speaker of a talk 

A talk has one speaker. The field controls edit the speaker, add a new speaker, or open the speakers in the Content > Records module:

The speaker of a talk with field controls

The speaker of a talk with field controls

EXT:my_extension/Configuration/TCA/Overrides/210-tx_myextension_talk-speaker.php 
<?php

use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;

defined('TYPO3') or die();

ExtensionManagementUtility::addTCAcolumns('tx_myextension_talk', [
  'speaker' => [
    'label' => 'my_extension.db:talk.speaker',
    'config' => [
      'type' => 'group',
      'allowed' => 'tx_myextension_speaker',
      'maxitems' => 1,
      'relationship' => 'manyToOne',
      // The speakers are stored in the same folder as the talks
      'elementBrowserEntryPoints' => [
        'tx_myextension_speaker' => '###CURRENT_PID###',
      ],
      'fieldControl' => [
        'editPopup' => [
          'disabled' => false,
        ],
        'addRecord' => [
          'disabled' => false,
        ],
        'listModule' => [
          'disabled' => false,
        ],
      ],
      'behaviour' => [
        'allowLanguageSynchronization' => true,
      ],
    ],
  ],
]);
$GLOBALS['TCA']['tx_myextension_talk']['types']['keynote']['columnsOverrides']['speaker']['config']['minitems'] = 1;
ExtensionManagementUtility::addToAllTCAtypes(
  'tx_myextension_talk',
  'speaker',
  '',
  'after:slug',
);
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