BIOS.

They recognized that PCs were the next big thing and needed one of their own. Large companies don't move fast, and IBM is certainly no exception, but they had to move fast now. So they took a bunch of off the shelf components that anyone else could have bought and called it their PC.

Everything except the BIOS. It regulated how the OS interacts with the hardware. Almost to the point where you could argue DOS isn't an OS at all, but just a thin command line layer over the BIOS, plus a simple minded file system.

Anyway, some people at Compaq make a cleanroom implementation of the BIOS and release an "IBM PC compatible". This quickly becomes the basis of everything we call a PC today. But IBM doesn't get to profit off it in the long run. They sold off their PC division decades ago.

The show "Halt and Catch Fire" has an excellent fictional example of the reverse engineering process.