Tactics (puzzles) are present in ALL chess games. It is just a matter of finding them.
To do that, our chess engine analyzes games played on Chess.com (and from other sources) to find games with blunders in them.
When you click 'Analyze' after a game, the engine processes the game and keeps an eye out for big evaluation swings (usually caused by blunders) If it spots a blunder, it will analyze the position using a sophisticated algorithm to parse the results, making sure there is only one clear winning line. This algorithm will also tag the problem with the appropriate themes (such as forks, skewers, etc).
This means that any of the positions you've played in your own games could possibly become puzzles! Just by playing you are helping us out!
Finally, we upload all these puzzles and let people try them out! Hundreds of new puzzles are added each week, and we review the puzzles that members report as flawed.