When I was a kid I had a game genie on the NES and Genesis, and a GameShark on the PSX. The Genesis one in particular (well maybe it was the GameShark, memory from that long ago is a bit fuzzy) had a VHS that explained how to make your own cheat codes. There were basically tools in the device that let you snapshot the memory at a specific point in time, then you could do an action, and get essentially a diff between the snapshots. By seeing what values changed, you could figure out what codes to set. The easiest was always infinite lives - snapshot, then kill yourself, and compare what changed. Do it a few times and you'll find the address for the life counter, so you'd set that to 99 and try it out til you got the right code and locked the value at 99.
The device really was cool as shit, it was my first introduction into doing anything with computers really, I spent hours creating my own codes as a kid.
IIRC cheat codes were often published in popular gaming magazines as well, which is how you found most of them in the world before widespread Internet & PC access. I'd assume that either manufacturers or some other party found common codes manually via similar methods to this.
The device really was cool as shit, it was my first introduction into doing anything with computers really, I spent hours creating my own codes as a kid.
IIRC cheat codes were often published in popular gaming magazines as well, which is how you found most of them in the world before widespread Internet & PC access. I'd assume that either manufacturers or some other party found common codes manually via similar methods to this.