Does requiring core/drupal load jQuery in custom script files? I am trying to understand a bit more about where and how jQuery is loaded in Drupal 8.

If I write a custom script file that will include some translation strings using Drupal.t(), I need to add core/drupal as a dependency.

I know we have a call for jQuery: core/jquery, but hoping to have a clear understanding.

Drupal core in places uses jQuery, does calling core/drupal include jQuery? When using Drupal.t()?

Is there a difference when doing so inside a custom behaviors wrapper?

The premise is if I write my scripts in vanilla js, will using core/drupal and/or behaviors still call any jQuery?
No, core/drupal depends only from core/drupalSettings; using it will not load core/jquery. The definition of those libraries is contained in core.libraries.yml, where  you can see their dependencies.

drupal:
  version: VERSION
  js:
    misc/drupal.js: { weight: -18 }
    misc/drupal.init.js: { weight: -17 }
  dependencies:
    - core/drupalSettings
  drupalSettings:
    suppressDeprecationErrors: true

drupalSettings:
  version: VERSION
  js:
    # Need to specify a negative weight like drupal.js until
    # https://www.drupal.org/node/1945262 is resolved.
    misc/drupalSettingsLoader.js: { weight: -18 }
  drupalSettings:
    # These placeholder values will be set by system_js_settings_alter().
    path:
      baseUrl: null
      scriptPath: null
      pathPrefix: null
      currentPath: null
      currentPathIsAdmin: null
      isFront: null
      currentLanguage: null
    pluralDelimiter: null