In the cassette tape player, a large safety precaution is the transformer. The transformer turns the 120 volts that is emitted from the outlet into just 9 volts. If the transformer weren't there, the 120 volts would have fried all of the circuit boards and might have even made it explode.
The buttons are connected to the tape feed mechanics which plays and pauses the cassette tape. Some, like the fast forward and rewind button, move the thin plates under the mechanics when pushed. They shift the tape of the cassette causing it to either rewind or fast forward. Similarly, the play button is also connected to the plates that also attaches to a spring, which pulls the plates back up.