How to create a library?

Er, The above advice is more for CI3.

For CI4, create libraries as classes inside some app folder, eg app/Libraries, and make sure that they are properly namepaced.

Eg:
<?php namespace App\Libraries\MyStuff;
class MyStuff {
}


and then load/reference it with
$mystuff = new App\Libraries\MyStuff();

and away you go.

Perfect, it works! Exemple: 

PHP Code:

<?php

namespace App\Libraries\Payment;

class Payment
{

    public function configName($name): string
    {
        return $name;
    }
} 


Another thing, it is recommended to use

PHP Code:

new \App\Libraries\Payment(); 


Or: 

PHP Code:

//This
use  App\Libraries\Payment;

class Home extends Controller
{
    public function index()
    {
        //This
        $payment = new Payment();

    {... }