Consider human brains as systems to integrate with.
What do you need to do to integrate with another system? Generally, you need a contract. You need to agree on the terms of your communication — protocols, primitives, messages, endpoints.
It’s kind of the same with people. In order to work together we need to be able to communicate and coordinate. We need common language and we need to agree on process.
In both cases, it takes work. And you might not get the design right on the first try.