We actually use all parts of our brain as most of our brain is active at all times. Brain activity happens without us actually having to do anything. Like breathing. It just happens.
Our brain works a bit like a computer hard drive. Everything we have seen, heard or done, is stored away in neat little files that we don’t access at all times. I am imagining color/date-coordinated labels here, a bit like in Stephen Kings ‘Dream Catcher’ where the character Jonesy has a secret ‘room’ in his brain with organized ‘memory’ files, where he can hide the information the alien possessing his body wants access to. But I’m digressing.