How does a laugh track actually get added to a sitcom episode?

[NealBinnom-Williams]

There are genuinely two distinct approaches here that often get confused with each other. Shows filmed in front of a genuine live studio audience capture real actual audience laughter live during filming, and editors then carefully sweeten and adjust that authentic real reaction, sometimes boosting a specific weaker laugh slightly or trimming down one that runs on for genuinely too long, but the underlying core laughter itself is real and comes from an actual present live audience.

Shows without any live studio audience present at all. Or those needing extra additional laughter beyond what an actual real crowd genuinely provided, use what is properly called a laugh library, a genuinely large archived collection of pre recorded laugh sound clips, originally captured from real actual audiences reacting to entirely different unrelated shows over the years.

A sound editor then carefully places specific individual laugh clips at precise chosen moments in a finished edit. Deliberately selecting the actual specific type of laugh, a quick single chuckle, a bigger sustained laugh, an audible group reaction, based specifically on how big or small a given joke actually is genuinely intended to land with a typical viewer.

The specific tool historically used to actually build and mix these laugh tracks became genuinely famous in its own right. A device called the Laff Box, originally built back in the 1950s specifically for the show The Hanna-Barbera and later widely used across countless other shows, containing tape loops of real audience reactions that a skilled dedicated technician could trigger and precisely blend together live during the actual final editing process.

So the honest answer genuinely depends entirely on the specific show. Real authentic audience reaction that just gets carefully sweetened, or entirely artificially constructed laughter pulled from an archived library, and audiences have actually gotten noticeably better over the years at consciously or unconsciously picking up on which specific version they are actually currently hearing

[FairDos96]

TLDR, some shows use real live studio audience laughter that just gets carefully edited and adjusted. Others use pre recorded laugh clips from an archived library, deliberately placed by a sound editor at specific chosen moments