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It could have just as easily been called e.g. The exe file is just a bootstrap that prepares the engine, and loads the actual mod (like Half-Life 2 or Counter Strike: Source), which in itself is a bunch of data and DLLs. You're never running the game - you're running the engine, telling it to select a given mod (note how the games are run like hl2.exe -game cstrike, for example).
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Source Engine games (all those you've mentioned) are really closer to what you'd usually call a ("total conversion") mod.
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