If the serve hits the receiving player (or their partner, when playing doubles) before touching down out of bounds, the point is the servers. The exception is if the serve is a let. In that case the point is not awarded to wither team and the server reserves.
If a player breaks the other player's serve in tennis, then the receiving player gets the game point, and that player will have the advantage of getting a two game lead over the opponent if the player can hold his or her own serve.
If at any point in the game where the receiving team has a break point (or Advantage in Ad-Scoring after a deuce), then it is called a break point.
In tennis and volley-ball you lose the point if you touch the net while the ball is in play.
If the tennis ball hit the player, even if the player is out of bounds, then the opponent gets the point.
The opponent's
I think it is badminton but that is just my point of view because I am a tennis player.
15?
No, if it does it is a point to the other player.
the player whose side the ball is on
The same thing as in ping-pong. If one player does not return the ball over the net, then the other player (or team) gets the point.
the aim of tennis is to hit the ball away from the other player so you can win the point and there for win games.
Then the opposing player must return the ball to compete for the current point.