The trick isn’t to look at the complete game code which will always have platform specific code but rather look at the tricks used by the game engines that are

The trick isn’t to look at the complete game code which will always have platform specific code but rather look at the tricks used by the game engines that are conceptual. Or in other words, look at the design rather than the implementation. This is true even for modern games, because you might implement something differently for a mobile game than you might for a high end PC.