Laravel Controller-Model Exception Handling structure with database transactions

First of all, for your example, you don't even need to use Transaction. You are performing just one query. so why do you need to rollback? Which query you want to rollback? A transaction should be used when you need a set of changes to be processed completely to consider the operation complete and valid. If the first one is successful, but any of the following has any error, you can rollback everything as if nothing was ever done.

Secondly, Lets come to the point good practice or best practice. Laravel suggest Thin controller and Thick model. So all of your business logic should be in model or even better in a repository. Controller will be act as a broker. It will collect data from repository or model and pass it to view.

Alternately, laravel provides some nice and convenient way to organize your codes. You can use Event and Observers for concurrent operation in your model.

Best practice is varies based on the knowledge and experience of the users. So who knows, the best answer for your question is yet to come.

I'd rather keep the controllers and any other part of the system that interacts with the models, as agnostic as possible about the inner workings of the model. So for instance I'd try to avoid being aware of QueryExceptions outside of the model and instead treat it as a plain PHP object whenever possible.

Also I'd avoid the custom JSON response structure and use HTTP statuses. If it makes sense, maybe the route for updating the user info returns the updated resource, or maybe a 200 OK is enough.

// UserModel.php
public function updateUserInfo($request)
{
    $username = $request['username'];
    $password = $request['password'];

    try {
        $this->connect()->beginTransaction();

        $this->connect()->table('users')->where('username', $username)->update(['password' => $password]);

        $this->connect()->commit();

        return $this->connect()->table('users')->where('username', $username)->first();
        // or just return true;
    } catch (\Exception $e) {
        $this->connect()->rollback();

        return false;
    }
}

// UserController.php    
public function updateUserInfo(UserInfoRequest $request, UserModel $userModel)
{
    $updated = $userModel->updateUserInfo($request->only(['username', 'password']));

    if ($updated) {
        return response($updated);
        // HTTP 200 response. Returns JSON of updated user.
        // Alternatively,
        // return response('');
        // (200 OK response, no content)
    } else {
        return response('optional message', 422);
        // 422 or any other status code that makes more sense in the situation.
    }

(Completely off-topic, I guess this is an example, but just in case, a reminder not to store plain text passwords.)

i don't understand, why you not watched Jeffry lesson, but for updating user you don't need try/catch section. you controller method:

public function update(UpdateUserRequest $request, User $user) : JsonResponse
{
   return response()->json($user->update($request->all()))
}

you request rules method:

public function rules(): array
{
    return [
        'username' => 'required|string',
        'password' => 'required|min:6|confirmed',
    ];
}

And you Exception Handler render method:

public function render($request, Exception $exception)
{
    if ($request->ajax() || $request->wantsJson()) {
        $exception = $this->prepareException($exception);

        if ($exception instanceof \Illuminate\Http\Exception\HttpResponseException) {
            return $exception->getResponse();
        } elseif ($exception instanceof \Illuminate\Auth\AuthenticationException) {
            return $this->unauthenticated($request, $exception);
        } elseif ($exception instanceof \Illuminate\Validation\ValidationException) {
            return $this->convertValidationExceptionToResponse($exception, $request);
        }

        // we prepare custom response for other situation such as modelnotfound
        $response = [];
        $response['error'] = $exception->getMessage();

        if (config('app.debug')) {
            $response['trace'] = $exception->getTrace();
            $response['code'] = $exception->getCode();
        }

        // we look for assigned status code if there isn't we assign 500
        $statusCode = method_exists($exception, 'getStatusCode')
            ? $exception->getStatusCode()
            : 500;

        return response()->json($response, $statusCode);
    }
    return parent::render($request, $exception);
}

Now, if you have Exception, Laravel give you in Json with status code != 200, else give success result!