The classloader is provided by Nextcloud and loads all your classes automatically. The only thing left to include by yourself are 3rdparty libraries. Those should be loaded in lib/AppInfo/Application.php.

PSR-4 autoloading

Since Nextcloud 10 there is a PSR-4 autoloader in place. 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/.