dbal.php

    'EXTCONF' => [
        'dbal' => [
            'handlerCfg' => [
                '_DEFAULT' => [
                    'type' => 'native',
                    'config' => [
                        'username' => '',        // Set by default (overridden)
                        'password' => '',        // Set by default (overridden)
                        'host' => '',            // Set by default (overridden)
                        'database' => '',        // Set by default (overridden)
                    ]
                ],
                'some_db' => [
                    'type' => 'native',
                    'config' => [
                        'username' => 'root',
                        'password' => '1234',
                        'host' => '127.0.0.1',
                        'database' => 'some_external_db',
                    ]
                ]
            ],
            'table2handlerKeys' => [
                'some_table' => 'some_db',
            ]
        ],
    ],