There are a few basic things a developer needs to do in a custom module.... SELECT, INSERT and DELETE queries are top of my list and this article demonstrates a few examples of SELECT and INSERT queries.

This code is within a controller that is part of a custom module. I won't explain how to build the module but this code is within a file called ReportController.php.

The basic file code is:

namespace Drupal\mycustommodule\Controller;

use Drupal\Core\Controller\ControllerBase;
use Drupal\Core\Database\Database;

class ReportController extends ControllerBase {

   // The main controller code goes in here

}

I put my queries into separate 'protected' functions which are called within the main 'public' function like this:

public function mainReportFunction() {
    $savedVideos = $this->getVideoIds();
}

I won't explain how the following functions work but they're basic SQL SELECT with a foreach() to return clean values or arrays.

// SELECT

protected function getVideoIds() {
  $select = Database::getConnection()->select('your_videos', 'yv');
  $select->addField('yv', 'yt_id');
  $entries = $select->execute()->fetchAll(\PDO::FETCH_ASSOC);

  $savedVideos = [];
  foreach ($entries as $v) {
      $savedVideos[$v['yt_id']] = true;
  }
  return $savedVideos;
}

// SELECT

protected function getNodeIdUsingVideoId($savedVideoId) {
  $select = Database::getConnection()->select('node__field_video_id', 'nfvid');
  $select->condition('nfvid.field_video_id_value', $savedVideoId, '=');
  $select->fields('nfvid', ['entity_id']);
  $select->range(0, 1);
  $entries = $select->execute()->fetchAll(\PDO::FETCH_ASSOC);

  if(isset($entries[0]['entity_id'])){
    return $entries[0]['entity_id'];
  }
  return false;
}

// INSERT

protected function addPlaylistEntry($ply) {
  $connection = \Drupal::service('database');
  $result = $connection->insert('video_playlist')
  ->fields([
    'title' => $ply['name']
    , 'playlist_id' => $ply['id']
    , 'description' => $ply['description']
    , 'thumbnail_high' => $ply['thumbnail_url']
  ])
  ->execute();

  if(!is_numeric($result)){
    \Drupal::logger('mycustommodule')->error('Error INSERT failed: '.$ply['id']);
  }
  return $result; // result is the row id
}

// DELETE

protected function deleteVideoById($vidId) {
  $delete = Database::getConnection()->delete('videos_table');
  $delete->condition('video_id', $vidId);
  $delete->execute();
  return true;
}