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.