Steam

Steam is Valve's online software delivery platform. The origins of Steam probably date to 1999 when Monica Harrington met with Amazon to pitch an online entertainment platform (Harrington, https://medium.com/@monicah428/the-early-days-of-valve-from-a-woman-inside-bf80c6b47961). Although no partnership with Amazon materialised, Valve clearly purused the idea itself. After beta testing in 2003 it went live on September 12th 2003 (https://store.steampowered.com/sale/steam20), though some Valve employees have account creation times from before the official go live date, for example https://steamcommunity.com/id/erikj. Each Steam application has a unique numeric appid. Each Steam application is made up of one or more depots. Depots store the actual application files can be used to separate out per-platform files for example i.e. Windows binaries can be in one depot and Linux binaries in another depot - the application can then be configured to download whichever depots are required. Updating a depot creates a new version of that depot, and each version has a manifestid.

Prior to the SteamPipe update in 2013 Steam delivered games in .gcf and .ncf files.

Downloading Steam depots

Open the Steam console: steam://open/console
Run the command download_depot <appid> <depotid> (you can get the appid and depotid from https://steamdb.info)
When it finishes downloading it will leave a message saying where it downloaded but it should be in steamapps/content/app_<appid>/depot_<depotid>

Older versions of depots can be downloaded by specifying the manifestid:
Run the command download_depot <appid> <depotid> <manifestid> (you can get the appid, depotid and manifestid from https://steamdb.info)

Steam Audio

The sound library that Steam uses has been open source: https://github.com/ValveSoftware/steam-audio