The kernel loads your program, and schedules its execution. This is basically the same as using an async framework and sending the entry point to your application.

If you're willing to extend a little, there are also signal handlers, or setting up callbacks in pthreads.

Again this isn't to be reductive, but to say this is a super common method of encapsulating irrelevant stuff. It doesn't seem to be a significant differentiation between "I'm a library" and "I'm a framework".