I’ve been trying to extend tt_address with a mm_relation with not much luck so far. Anything I can find online is just for a regular relation, where the ID is saved directly into the same table, not an extra _mm table.
The backend looks fine so far, the way I want it with a selectMultipleSideBySide renderType. It also saves the relations to the database.

Typo3 12.4.16 // TT_Address 8.1.0 // extender 10.1.1

…/extensions/wiwa/Configuration/TCA/Overrides/tt_address.php

<?php
defined('TYPO3') or die();

use TYPO3CMSCoreUtilityExtensionManagementUtility;

(static function () {
    ExtensionManagementUtility::addTCAcolumns('tt_address',
        [
        'product_groups' => [
            'exclude' => true,
            'label' => 'LLL:EXT:wiwa/Resources/Private/Language/locallang_db.xlf:tx_wiwa_domain_model_product.product_group',
            'description' => 'Auswahl der WIWA-Produktkategorie',
            'config' => [
                'type' => 'select',
                'renderType' => 'selectMultipleSideBySide',
                'foreign_table' => 'tx_wiwa_domain_model_productgroup',
                'MM' => 'tt_address_productgroup_mm',                
                'default' => 0,
                'size' => 10,
                'autoSizeMax' => 30,
                'maxitems' => 3,
                'multiple' => true,
                'fieldControl' => [
                    'editPopup' => [
                        'disabled' => false,
                    ],
                    'addRecord' => [
                        'disabled' => false,
                    ],
                    'listModule' => [
                        'disabled' => true,
                    ],
                ],
            ],
           ],
          ],
    );

    ExtensionManagementUtility::addToAllTCAtypes(
        'tt_address',
        'product_groups',
        '',
        'before:name'
    );
})();        
I followed Extend models of tt_address
.../extensions/wiwa/ext_tables.sql
CREATE TABLE tt_address_productgroup_mm (
    uid_local int(11) unsigned DEFAULT '0' NOT NULL,
    uid_foreign int(11) unsigned DEFAULT '0' NOT NULL,
    sorting int(11) unsigned DEFAULT '0' NOT NULL,
    sorting_foreign int(11) unsigned DEFAULT '0' NOT NULL,

    PRIMARY KEY (uid_local,uid_foreign),
    KEY uid_local (uid_local),
    KEY uid_foreign (uid_foreign)
);
CREATE TABLE tt_address (
    product_groups int(11) unsigned DEFAULT '0',     
);  

…/extensions/wiwa/Classes/Extending/Domain/Model/Address.php

<<?php

declare(strict_types=1);

namespace ReplicareWiwaExtendingDomainModel;

class Address extends FriendsOfTYPO3TtAddressDomainModelAddress
{

    /**
     * productGroups
     *
     * @var TYPO3CMSExtbasePersistenceObjectStorage<ReplicareWiwaDomainModelProductGroup>
     */
    protected $productGroups = null;
    
    /**
     *
     * @return void
     */
    public function __construct()
    {
        $this->productGroups = $this->productGroups ?: new TYPO3CMSExtbasePersistenceObjectStorage();
    }    
    
    /**
     * Adds a productGroup
     *
     * @param ReplicareWiwaDomainModelProductGroup $productGroup
     * @return void
     */
    public function addProductGroup(ReplicareWiwaDomainModelProductGroup $productGroup)
    {
        $this->productGroups->attach($productGroup);
    }

    /**
     * Removes a productGroup
     *
     * @param ReplicareWiwaDomainModelproductGroup $productGroupToRemove The productGroup to be removed
     * @return void
     */
    public function removeProductGroup(ReplicareWiwaDomainModelProductGroup $productGroupToRemove)
    {
        $this->productGroups->detach($productGroupToRemove);
    }

    /**
     * Returns the productGroups
     *
     * @return TYPO3CMSExtbasePersistenceObjectStorage<ReplicareWiwaDomainModelProductGroup> $productGroups
     */
    public function getProductGroups()
    {
        return $this->productGroups;
    }

    /**
     * Sets the productGroups
     *
     * @param TYPO3CMSExtbasePersistenceObjectStorage<ReplicareWiwaDomainModelProductGroup> $productGroups
     * @return void
     */
    public function setProductGroups(TYPO3CMSExtbasePersistenceObjectStorage $productGroups)
    {
        $this->productGroups = $productGroups;
    }

}

Changes in ext_localconf.php are not needed anymore: Changelog extender
and I have configured extender in services.yaml following the instructions in the extender manual.

# Configuration/Services.yaml
services:
  _defaults:
    autowire: true
    autoconfigure: true
    public: false


    
  ReplicareWiwaExtendingDomainModelAddress:
    tags:
      -
        name: 'extender.extends'
        class: FriendsOfTYPO3TtAddressDomainModelAddress    

The problem I’m having now is getting these relations in my fluid templates. The variable location.product_groups just contains the integer number of related objects, instead of the list of objects.
Extbase Variable Dump
How can I get the product_groups that are associated with that location in fluid, with an m:m relation?