To get list of methods you can run on certain types of entities you can get the list by loading this entity and then running get_class_methods on it so for example
$coupon = \Drupal::entityTypeManager()->getStorage('commerce_promotion_coupon')->load(3);
ksm(get_class_methods($coupon));
will list all the methods you can run on commerce_promotion_coupon entity.
Some other ways to get list of methods would be by namespace, so you can get methods for any classes, even vendor projects
ksm(get_class_methods('PhpOffice\PhpSpreadsheet\Reader\Xls'));
or the nices way to get output would be
$class = new ReflectionClass('PhpOffice\PhpSpreadsheet\Collection\Cells');
$methods = $class->getMethods(ReflectionMethod::IS_PUBLIC);
ksm($methods);