How do game updates work?

When a developer publishes a newer build, the game's button in your library changes from Play to Update. Nothing is downloaded until you press it.

Updates are incremental where the game supports it: the launcher compares what you have on disk with what the new build contains and downloads only the files that actually changed, which usually makes a patch a small fraction of the full game. Every downloaded file is checked against its expected fingerprint, and the new files are only swapped into place once all of them have arrived — so a crash or a lost connection mid-update cannot leave you with a half-patched game.