This mean we can use bootstrap or jquery from a client script without loading anything?

jQuery is available globally using the variable jQuery. They used to have jQuery UI loaded as well but that was removed a couple releases ago. You can also use Ext.js but it's also an antiquated version, and of course all at your own risk because they don't give any promises about the libraries being available.

Would it be possible to embed your own javascript in every page to check for jQuery and bootstrap it if not found?