They take notes after the hand is over either on their cellphones or in a small notebook. Poker, like chess, has a shorthand that enables a person to capture a hand pretty easily with a few anacronyms and numbers. It is not hard to recount a hand with a few notes and a video to jog your memory.
The best way to track live poker stats is to first write them down on your phone while at the table. When you get home put them into an Excel poker sheet, or just through an Excel app on your phone right away. This makes it simple to get instant reports and allows for more in-depth analysis.