The classloader is provided by Nextcloud and loads all your classes automatically. See the composer section if you want to include and autoload 3rd party libraries.

PSR-4 autoloading

Nextcloud uses a PSR-4 autoloader. The namespace \OCA\MyApp is mapped to /apps/myapp/lib/. Afterwards normal PSR-4 rules apply, so a folder is a namespace section in the same casing and the class name matches the file name.

If your appid can not be turned into the namespace by uppercasing the first character, you can specify it in your appinfo/info.xml by providing a field called namespace. The required namespace is the one which comes after the top level namespace OCA\, e.g.: for OCA\MyBeautifulApp\Some\OtherClass the needed namespace would be MyBeautifulApp and would be added to the info.xml in the following way:

<?xml version="1.0"?>
   <!-- other options here ... -->

A second PSR-4 root is available when running tests. \OCA\MyApp\Tests is thereby mapped to /apps/myapp/tests/.