code: public function execute()
      {
          $result = $this->resultJson->create();
  
          try {
              $snippet = $this->getRequest()->getParam('snippet_id');
  
              $read = $this->filesystem->getDirectoryRead(DirectoryList::VAR_DIR);
              $snippetPath = $read->getRelativePath(Config::CUSTOM_SNIPPET_PATH . $snippet);
  
              if ($read->isExist($snippetPath)) {
                  $explodeId = explode('.', $snippet, -1);
                  $snippetParts = explode('_', $explodeId[0], 3);
                  $type = $snippetParts[0];
                  $priority = $snippetParts[1];
                  $name = $snippetParts[2];
                  $content = $read->readFile($snippetPath);
              } else {
                  throw new LocalizedException(__('Custom snippet not found.'));
              }
  
              return $result->setData([
                  'status'    => true,
                  'type'      => $type,
                  'priority'  => $priority,
                  'name'      => $name,
                  'content'   => $content,
                  'original'  => $snippet
              ]);
          } catch (\Exception $e) {
              return $result->setData([
                  'status'    => false,
                  'msg'       => $e->getMessage()
              ]);
          }
      }
docs: Get custom snippet
  
  @return $this|\Magento\Framework\App\ResponseInterface|\Magento\Framework\Controller\ResultInterface