Semantically, most OOP models[1] (even C++) involve "messages" that are "sent" to an object. "Methods" are "how do I handle this kind of message".
This usually resolves to a function call because it's the easiest and most sensible way to do it.
Objective-C is more explicit about it due to Smalltalk heritage. Some languages model objects as functions (closures) that one calls with the "message" (an old trick in various FP languages is to implement a trivial object system with closures for local state).
[1] Arguably CLOS with Generic Functions can be seen as outlier, because the operation becomes centerpiece, not the object.
