GST on purchase of a Flat

Sir,
 I purchased one flat total cost of Rs. 48.98 lacs which was under construction in dec 2018 and got registered.  
During the registration time the builder collected gst of Rs.3.92 lac s (8% on total cost including land cost)
My flat completed recently and I occupied in Jan 2021.
Now my question is 
1) whether the builder collected GST correctly.
2) whether I can get the benefit of revised / reduced rate of gst after my registration.