Deny access by URI to the items in the database if it's don't belonged to the current user.

There's actually some free videos on this in this series, authentication and authorization: https://laracasts.com/series/laravel-from-scratch-2018

But that's what the auth id is for.

Use that series along with the newer 6.* scaffolding.

https://laravel.com/docs/6.x/authentication#retrieving-the-authenticated-user

Note also, there's a entire chapter on both in the docs.

Also authorization is a fairly large learning curve, hard to give a month lesson in a forum post.

You may want to look at https://github.com/spatie/laravel-permission this may be easier to get up and running. Note, still a big learning curve however.

Look into either:

Option 1: Using Middleware

• https://laravel.com/docs/master/middleware

Option 2: Redirect in the Controller

The controller you've got set up presumably has an $id value that is passed to the route or that is otherwise accessible. Add a statement like:

// Toward the start of the controller. 
// This Eloquent query syntax might be off. Replace with your query.
$product = \Auth::user()->with('products')->find($id);
if ( !$product ) { 
    // Redirect the user back to the previous page.
    // The withErrors value can be used/displayed as described in 
    // the Stack Overflow thread linked below
    return redirect()->back()->withErrors(['You must purchase this product to access this webpage.']);

    // Or alternatively
    die("Error: You do not have access to this product. Click <a href='#'>here</a> to buy it/return to home/other."); 
} 

// Put the rest of the controller below this 

See also:

• https://stackoverflow.com/questions/19838978/laravel-redirect-back-with-message