listening to ATC on United Airlines

I'm not a pilot, just your typical passenger. I don't fly United often,
so I just discovered that they put the ATC on the inflight audio. I was
totally fascinated and want to learn some more. Is there a website that
has some basic ATC protocols? Not some huge reference manual, just the
common phrases that you hear over a commercial flight's radio.

A few questions I had while listening:

- What happens if 2 people try to talk at the same time? Do you hear
both voices or neither? How do they avoid talking over each other?

- It seems that there is no authentication/security, the parties just
identify themselves. Have there been cases of troublemakers making
false impersonations?

- They ask pilots their speed, altitude, and heading a lot. Can't the
transponder send that information?

- Do any other airlines put ATC on the inflight audio?