In first tutorial we introduce the creation of system configuration , in this course  we explain how set and get the value (play with configuration).

1 – Set Value Configuration :

You can set the value of specific field  inside  stores configuration , by using the interface   \Magento\Framework\App\Config\MutableScopeConfigInterface  , (magento2 by calling this interface with technique of preference give you as result a MutableScopeConfig class instance)
inject the class to controller or helper or block or anywhere :

  
    /**
     * Config configResource
     *
     * @var configResource
     */
   protected $configResource;
  /**
   * @param Context $context
   * @param MutableScopeConfigInterface  $confige
     */
    public function __construct(
        Context $context,
        \Magento\Framework\Json\Helper\Data $jsonHelper,
        \ Magento\Framework\App\Config\MutableScopeConfigInterface $config
        
    ) {
        $this->config  = $config;
        parent::__construct($context);
    }
 

 And you can use the function saveConfig :

 
$this->config->setValue('ibnab_asterisk_control/apioptions/api_key', $api_key,\Magento\Framework\App\Config\ScopeConfigInterface::SCOPE_TYPE_DEFAULT,0);
 

'ibnab_asterisk_control/apioptions/api_key' : the path to you field configuration as (section/group/id)
$api_key : is the value to set
\Magento\Framework\App\Config\ScopeConfigInterface::SCOPE_TYPE_DEFAULT : the scope to use to determine config value, e.g., 'store' or 'default' , here we use the const SCOPE_TYPE_DEFAULT = 'default';
0 : the scopeCode

    <preference for="Magento\Framework\App\Config\MutableScopeConfigInterface" type="Magento\Framework\App\MutableScopeConfig" />

2 – Get Value Configuration :
You can get the value of specific field  inside  stores configuration , by using the interface \Magento\Framework\App\Config\ScopeConfigInterface , yes DI give you the real class responsible
Magento\Framework\App\Config  :

  
    /**
     * Config configResource
     *
     * @var configResource
     */
   protected $configResource;
   /**
   * @param Context $context
   * @param ScopeConfigInterface   $confige
     */
    public function __construct(
        Context $context,
        \Magento\Framework\Json\Helper\Data $jsonHelper,
        \ Magento\Framework\App\Config\ScopeConfigInterface  $config
        
    ) {
        $this->config  = $config;
        parent::__construct($context);
    }
 

 
   $this->apiKey = $this->$config->getValue('ibnab_asterisk_control/apioptions/api_key', \Magento\Store\Model\ScopeInterface::SCOPE_STORE,$scopeCode);
 

almost the same technique .

3 – Other thinks:
The module Magento_Config is a big player , magento2 in many place use  \Magento\Config\Model\ResourceModel\Config  for saving the configuration , is the  resource responsible of saving getting configuration value from db you can use directly :

  
    /*
     *var configResource
     */
    protected $configResource;
    */
    public function __construct(
        Context $context,
        \Magento\Config\Model\ResourceModel\Config $configResource
         
    ) {
        $this->configResource  = $configResource;
        parent::__construct($context);
    }
 

and you can use the method deleteConfig($path, $scope, $scopeId):

$this->configResource->deleteConfig('ibnab_asterisk_control/apioptions/api_key', ScopeConfigInterface::SCOPE_TYPE_DEFAULT,$scopeId);
for deleting the the row from db...

4 – About scopes :
you can use \Magento\Store\Model\ScopeInterface for getting other scope values for example:
\Magento\Store\Model\ScopeInterface::SCOPE_STORE the result is  store.
and have other constants :

  
    const SCOPE_STORES = 'stores';
    const SCOPE_WEBSITES = 'websites';
    const SCOPE_STORE   = 'store';
    const SCOPE_GROUP   = 'group';
    const SCOPE_WEBSITE = 'website';
 

or for default:
ScopeConfigInterface::SCOPE_TYPE_DEFAULT

5 – Getting current scope and scope code (id) from URL:

inject \Magento\Framework\App\RequestInterface $request and \Magento\Store\Model\StoreManagerInterface $storeManager with di :

 
    /**
     * @var \Magento\Store\Model\StoreManagerInterface
     */
    protected $storeManager;
    /**
     * @var \Magento\Framework\App\RequestInterface
     */
    protected $request; 
    public function __construct(
        \Magento\Framework\App\Helper\Context $context,
        \Magento\Store\Model\StoreManagerInterface $storeManager,
        \Magento\Framework\App\RequestInterface $request
    ) {
        $this->storeManager = $storeManager;
        $this->request = $request;
        parent::__construct($context);
    }
 

and use inside getCurrentScope() for example :

 
    public function getCurrentScope()
    {
       $scopeType = \Magento\Framework\App\Config\ScopeConfigInterface::SCOPE_TYPE_DEFAULT;
       $scopeCode = 0;
       if($this->request->getParam(\Magento\Store\Model\ScopeInterface::SCOPE_STORE))
       {   
        $scopeType = ScopeInterface::SCOPE_STORE;
        $scopeCode = $this->storeManager->getStore($this->request->getParam(\Magento\Store\Model\ScopeInterface::SCOPE_STORE))->getCode();
       }elseif($this->request->getParam(\Magento\Store\Model\ScopeInterface::SCOPE_WEBSITE))
       {
        $scopeType = ScopeInterface::SCOPE_WEBSITE;
        $scopeCode = $this->storeManager->getWebsite($this->request->getParam(\Magento\Store\Model\ScopeInterface::SCOPE_WEBSITE))->getCode();
       }   
       else
       {
        $scopeType = \Magento\Framework\App\Config\ScopeConfigInterface::SCOPE_TYPE_DEFAULT;
        $scopeCode = 0;       
       }
       return [$scopeType,$scopeCode];
    }
 

Here we get the current scope and cuurent scope code (you can get the id) , you can use this params for getting and setting the value of cuurent scope :

 
$arrayCurrentScope = $this->getCurrentScope();
   $this->apiKey = $this->$config->getValue('ibnab_asterisk_control/apioptions/api_key', $arrayCurrentScope[0],$arrayCurrentScope[1]);