In the early 90s, Ford used a slightly special version of Intel's 8096 chip for the heart of its ECU.
You can find schematics and back-engineered source code for those boxes on the net with a little searching.
Regular 8096 toolchains would probably work okay, mostly, but Intel never gave them away.
In those days of yore, the program was stored in an EPROM, which could be read by a generic reader.
Nowadays, I suspect everyone is using completely special 'System On Chip' setups that are not easily accessed, and may bear no resemblance to any commodity chips.
Why too broad of question? There are quite a few outfits that do automotive ECU repairs, they have to get test instrumentation and service literature from some place. And they need something to simulate the engine sensor signals to test it for proper operation. I know most problems with automotive control modules are caused from aged electrolytic capacitors and its likely that a very high percentage of the repairs are just swapping them out. Still the unit needs to be tested some how. I am truly amazed how difficult it is to find service information on any automotive electronic black box. Even the radios on some early 90's cars seems to be very scarce.
So, you want to know about every PCM every car manufacturer has ever made?
Generally, I'd think the manufacturers sell the information to the repair places since the newer stuff is way too complex to reverse engineer. Otherwise, companies are reverse engineering them and I doubt they'd want to share the info and lose their competitive advantage.
You make the test jigs, the ones you can anyways. Once again, the newer PCM's are way too complex to easily simulate the operation.