How to restrict routes or url in laravel 5.7 to each user?

As you mentioned before, it is best to use a group, but in addition to that, it is also good that you add an alias, a namespace and a specific middleware to validate the admin.

All this because generally the controllers and routes of the admin must be separated from the user routes, and the middleware because the user admin is validated different from the normal user

Everything should be more or less like this

Route::namespace('Admin')
     ->prefix('admin')
     ->name('admin.')
     ->middleware(['auth', 'admin'])
     ->group(function () 
    {
      /*
      Route::get('/', function () {
          // 
      });
      */   
    });

In this way all the routes are like this:

mipagina.com/admin/registrar-product the admin is added in an "automatic" way

The names of the routes are as follows:

admin.registerProduct

And the path of the controllers like this:

App \ Http \ Controllers \ Admin

Remember that you must create the middleware here you can find the documentation about it link

Here you can find an interesting article about it Laravel Route Tips to Improve Your Routing

You do not specify the version of laravel, but in any case, the groups of routes are the most appropriate. ( link )

Example of routes for admin exclusively:

Route::group(['prefix' => 'admin',  'middleware' => 'auth'], function() {
    /*
    Route::get('/', function () {
        // 
    });
    */   
});

Greetings