Rey knows how to fly because shes been around ships all her life, she knows the falcon because Unkar Putt owns it and shes probably been in it many times. In Star Wars, knowing how to fly is basically knowing how to drive.
She knows the Falcon inside out, yet claims that it’s ‘garbage’ before later teaching Han about compressors inhibiting the hyperdrive and auxiliary fuel pumps etc etc. It’s odd that someone with such intimate mechanical knowledge of the galaxy’s most famous hot rod would be so disparaging of it before boarding.
She doesn’t know it’s the Falcon at the time. Before meeting Han, it’s just some old hunk of junk freighter Unkar Plut keeps around under a tarp.