How to use register_setting()

The function register one option or a settings group. So yes, you register the group of a section and use all fields inside this group because the group store all settings fields as array in one item.

register_setting(
    '_example_plugin_settings',
    '_example_object_settings',
    array(
        'type'         => 'object',
        'default'      => array(
            'some_str' => 'A',
            'some_int' => 3,
        ),
    )
);
add_action( 'admin_init', 'example_register_settings' );
/**
 * One settings group with two items.
 */
function example_register_settings() {

  register_setting(
    '_example_plugin_settings',
    '_example_object_settings',
    '_validate_example_plugin_settings'
  );

  add_settings_section(
    'section_one',
    'Section One',
    '_section_one_text',
    '_example_plugin'
  );

  add_settings_field(
    'some_text_field',
    'Some Text Field',
    '_render_some_text_field',
    '_example_plugin',
    'section_one'
  );
  add_settings_field(
    'another_number_field',
    'Another Number Field',
    '_render_another_number_field',
    '_example_plugin',
    'section_one'
  );

}

If you get the settings entry, use get_option('_example_plugin_settings'), and you get all content of this item, like

$options = get_option('_example_plugin_settings');
print_r($options['some_text_field']);

The documentation of the function also has helpful examples.