Not exactly the question you were asking, but there are also SDKs for closed source software. You can get a library, or just an interface definition you adapt to. It can be frustrating when you cannot peek a layer deeper into the system, and takes head banging, but it's a thing. Often, if you are a significant enough client, you can get consulting or guidance from the devs at the other end.

Nowadays a lot more business software is open source (at least partially), because it increases adoption. People found that when you remove the stops, others will flock and build stuff around.