Redirects to Frontend

Tools::redirect('index.php')

Tools::redirectLink('index.php')

$this->canonicalRedirection('??')

Redirects to Backend

Tools::redirectAdmin('urlHere')
Tools::redirectAdmin($this->context->link->getAdminLink('AdminControllerHere'))
Tools::redirectAdmin(Context::getContext()->link->getAdminLink(AdminDashboard::class))

How to generate an URL

See this article on how to generate an URL in Prestashop