Generate models in common when shared between frontend and backend. Eg: common/models/Content.php:

<?php
 
namespace common\models;     // <<-- Namespace for common (shared) models for frontend and backend
 
use Yii;
 
...
class Content extends \yii\db\ActiveRecord
{
    ...
}

When creating a controller for frontend sharing common models, use this template (eg: frontend/controllers/ContentController.php:

<?php
 
namespace frontend\controllers;     // <<-- Frontend controller
 
use Yii;                            // For Yii application
use common\models\Content;          // <<-- Common model
use yii\web\Controller;             // For base Controller class
use yii\web\NotFoundHttpException;
use yii\filters\AccessControl;
use yii\filters\VerbFilter;
 
class ContentController extends \yii\web\Controller
{
    public function actionIndex()
    {
        $model = new Content();
 
        if ($model->load(Yii::$app->request->post())) {
            if ($model->validate()) {
                // form inputs are valid, do something here
                return;
            }
        }
 
        return $this->render('index', [
            'model' => $model,
        ]);
    }
 
}

Create view for frontend. Eg: frontend/views/content/index.php:

<?php
 
use yii\helpers\Html;
use yii\widgets\ActiveForm;
 
/* @var $this yii\web\View */
/* @var $model common\models\Content */
/* @var $form ActiveForm */
?>
<div class="content-index">
 
    <?php $form = ActiveForm::begin(); ?>
 
        <?= $form->field($model, 'title') ?>
        <?= $form->field($model, 'category_id') ?>
        <?= $form->field($model, 'hits') ?>
        <?= $form->field($model, 'rating_sum') ?>
        <?= $form->field($model, 'rating_count') ?>
        <?= $form->field($model, 'show_title') ?>
        <?= $form->field($model, 'show_intro') ?>
        <?= $form->field($model, 'show_image') ?>
        <?= $form->field($model, 'show_hits') ?>
        <?= $form->field($model, 'show_rating') ?>
        <?= $form->field($model, 'content_type_id') ?>
        <?= $form->field($model, 'featured') ?>
        <?= $form->field($model, 'ordering') ?>
        <?= $form->field($model, 'status') ?>
        <?= $form->field($model, 'created_by') ?>
        <?= $form->field($model, 'updated_by') ?>
        <?= $form->field($model, 'introtext') ?>
        <?= $form->field($model, 'fulltext') ?>
        <?= $form->field($model, 'publish_up') ?>
        <?= $form->field($model, 'publish_down') ?>
        <?= $form->field($model, 'created_at') ?>
        <?= $form->field($model, 'updated_at') ?>
        <?= $form->field($model, 'tags') ?>
        <?= $form->field($model, 'intro_image') ?>
        <?= $form->field($model, 'intro_image_float') ?>
        <?= $form->field($model, 'main_image') ?>
        <?= $form->field($model, 'main_image_float') ?>
 
        <div class="form-group">
            <?= Html::submitButton(Yii::t('app', 'Submit'), ['class' => 'btn btn-primary']) ?>
        </div>
    <?php ActiveForm::end(); ?>
 
</div><!-- content-index -->

To create links from the backend application to the frontend application, and vice versa, create separate URL manager rules by naming it differently:

Eg: file backend/config/main.php

return [
    'components' => [
        'urlManager' => [
            // Modify current Yii::$app->urlManager.
            // Here is your normal backend url manager config.
            'enablePrettyUrl' => true,
            'showScriptName' => false,
            'rules' => []
        ],
        'urlManagerFrontend' => [
            // Create a new urlManager to point to frontend (Yii::$app->urlManagerFrontend).
            // Here is your frontend URL manager config.
            'class' => 'yii\web\UrlManager',
            'baseUrl' => 'a/frontend/web',
            'enablePrettyUrl' => true,
            'showScriptName' => false,
            'rules' => []
        ],
 
    ],
];

Create a .htaccess file in each web folders (/frontend/web and /backend/web) to contain this:

RewriteEngine on
 
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d 
 
RewriteRule . index.php

After this, you can get an URL pointing. Eg: In a backend view, in order to link to frontend, use this:

// Some route on frontend. 
echo Yii::$app->urlManagerFrontend->createUrl('customer/index');
// OR frontend baseUrl:
echo Yii::$app->urlManagerFrontend->createUrl(''); 

Add a $frontendUrl global variable to basic/config/params.php or common/config/params.php:

Simple add a key and value pairs:

return [
    ...
    'frontendUrl' => 'http://www.example.com/frontend/web',
];

You can then use that variable anywhere in the system:

<?= Yii::$app->params['frontendUrl'] ?>