The T-90 is a souped up T-72. Modern T-72 variants use the same V-92S2F engine as the T-90 variants. There is a huge degree of commonality between both tanks. The main difference is the turret.
It isn't, though. It might have started life as such but later on they became different tanks. You would call an M60 the same as an M47. Hull armor layout and other things are different from the T-72 to T-90 which is what allows the T-90M to take the longer 3BM60 rounds than the T-72B3M cannot.
But wait then we have the T80 which has a completely different automatic loader and all the other components are different as it was designed in Ukraine by a completely different manufacturer.
Common misconception on the basis of the T-80's being an extension of the hi tech-lo tech mix trend of T-64/T-62. The T-80 is a Russian design, made from the ground up by Leningrad Kirov Plant and that at some point the Soviet Union made Kharkov produce the UD variant.