Why could Tetsuo's hand reproduce many flesh, muscles and veins using his ESP?
In the manga, it is explained that the power that gives Tetsuo his psychokinetic abilities is trying to break free, as his body is no longer enough to contain it. To compensate for this, his body is trying to absorb material from all around him. As for why he could produce additional matter, the power trying to break free might be partially converting energy into additional mass during his breakdown, allowing him to generate additional flesh.