From the documentaries of most devs it seems they usually had some sort of set of studio tools. Then each game was coded from scratch. There was some re-use o

From the documentaries of most devs it seems they usually had some sort of set of studio tools. Then each game was coded from scratch. There was some re-use of code between games. But not really in any sort of lib but more of a 'hey I have a function that does xyz'. Then they would use in place or rewrite to match their internals. Usually if it was the same dev you would see that a lot. But sometimes in a studio you would see some sharing of code. These days it seems most studios buy/download one of the big engines and start from there. They will also re-use an engine from game to game even if it is not what the original engine was built for. The game engines really came to the forefront when you are targeting 4 different platforms and need that abstraction away from each platforms API/hardware. If you are targeting 1 platform it may be just a matter of fill this memory region out correctly and tweak an interrupt and let the hardware do the rest. Not much for an engine to do.