MacOS (ie NeXTstep) was built from the ground up to be an open and extremely extensible OS.

There were countless ways to add in some 3rd party extension or hook. Since there were multiple runtimes to access your software, it was actually an impressive technical feat at the time to have it all work together seamlessly.

Java, classic Mac, X11, and the NeXTstep codebases all ran together without issue due to several of the kernel's extensible entry-points.

On top of that, the platform had APIs that let apps talk to each other without issue.

However, little by little, Apple has backtracked on that philosophy and continues to close the system down. Quite a fascinating journey.