The CRUD and search models of web APIs are implemented through a set of *RepositoryInterface interfaces, found in the <VendorName>/<ModuleName>/Api/<EntityName>RepositoryInterface.php files.

The majority of these repository interfaces define a specific set of common methods:

• save
• get
• getById
• getList
• delete
• deleteById

The data type that flows through these methods follows a certain pattern, where each entity passing through an API has a data interface defined in a <VendorName>/<ModuleName>/Api/Data/<EntityName>Interface.php file.

Let's take a closer look at <MAGENTO_DIR>/module-cms/Api/BlockRepositoryInterface.php:

interface BlockRepositoryInterface
{
    public function save(
        \Magento\Cms\Api\Data\BlockInterface $block
    );
    
    public function getById($blockId);
    
    public function getList(
  ...