(7) ‘controller’ means the natural or legal person, public authority, agency or other body which, alone or jointly with others, determines the purposes and means of the processing of personal data; where the purposes and means of such processing are determined by Union or Member State law, the controller or the specific criteria for its nomination may be provided for by Union or Member State law;
(8) ‘processor’ means a natural or legal person, public authority, agency or other body which processes personal data on behalf of the controller;
Generally speaking, site X is the data controller and the third party JS providers are the data processors. GDPR applies to both, with the controller being the party primarily responsible for ensuring compliance.