In several of the cases listed here, the game's developers released the source code expressly to prevent their work from becoming abandonware. Such source code is often released under varying (free and non-free, commercial and non-commercial) software licenses to the games' communities or the public; artwork and data are often released under a different license than the source code, as the copyright situation is different or more complicated. The source code may be pushed by the developers to public repositories (e.g. SourceForge or GitHub), or given to selected game community members, or sold with the game, or become available by other means. The game may be written in an interpreted language such as BASIC or Python, and distributed as raw source code without being compiled; early software was often distributed in text form, as in the book BASIC Computer Games. In some cases when a game's source code is not available by other means, the game's community "reconstructs" source code from compiled binary files through time-demanding reverse engineering techniques.
Once games, or software in general, become an obsolete product for a company, the tools and source code required to re-create the game are often lost or even actively destroyed and deleted. For instance, with the closure of Atari in Sunnyvale, California in 1996, the original source codes of several milestones of video game history such as Asteroids and Centipede were all thrown out as trash.
Using the techniques listed above within a "bottom-up" development methodology process, the re-created source-code of a game is able to replicate the behavior of the original game exactly, often being "clock-cycle accurate", and/or "pixel-per-pixel accurate". This approach is in contrast to that used by game engine recreations, which are often made using a "top-down" development methodology, and which can result in duplicating the general features provided by a game engine, but not necessarily an accurate representation of the original game.
A Remake is a game where the executable and sometimes the assets as well are remade open source. Some of these games aren't exact remakes but evolution of original ones, which were eventually open sourced. A Clone is a game which is very similar to or heavily inspired by a game or series. An Official project is the official source code release for a game that was formerly closed-source, maintained by the original creators and has minimal changes. A Similar game is one which has similar gameplay but is not a clone. A Tool is not a game, but something that assists in playing or modding the game, such as a high resolution patch, or resource extractor. 2b1af7f3a8