Gst rate on contractors

Please inform me about the applicability of Gst on contractors and their ITC claim on the goods that they supply to Govt departments.
for eg . The contractors who provide wardrobes to departments has used material as plywood and nails and hardware for its manufaturing which are in itself taxable at different rates then at what rate they will charge Output gst (whether they will generate item wise bill or jus service bill) and how can they claim ITC