Good questions.i've never really technically looked into these myself -- i have always just kind of...traded...
I don't think the 'winner' takes money Directly from the 'loser'...think of it instead as one big pool...where all the bets are thrown in, and taken out/paid out accordingly.
I was one time witness from what happened at a Forex desk from a very big US broker. I was surprised that in general 50% of the people went short while at the same time 50% went long. I could follow these bizar trades for a full day.
Some went short to close a long, while others really went short. The same for longs. Holding times were very different too, so the reason to trade was different too. There are always dozens of reasons to act. And what might seem for you as a loss can in reality be a winner, and vice versa. You never know the whole story (when they got in?) so it is impossible to see who is winning and who is losing. It can even be a controlled loss that was more kind of an insurance against bigger risk.
Don't break your head on others, break your head on your own trades. That's all that matters.