How to upload a file in Drupal 8 using FAPI

  $form['file'] = [
    '#type' => 'managed_file',
    '#title' => t('File'),
    '#upload_location' => 'public://file',
    '#upload_validators' => [
      'file_validate_extensions' => ['csv'],
    ],
    '#default_value' => array(2)
  ];
  
  
  Handle the saving the file as a permanent file into the file_managed table. So add this to your submit handler.
  
  use Drupal\file\Entity\File;

  $fid = $form_state->getValue(['file', 0]);
  if (!empty($fid)) {
    $file = File::load($fid);
    $file->setPermanent();
    $file->save();
  }