Trimmed down https://stackoverflow.com/a/59596600 to make it even more minimal

minimal.c

// based on https://stackoverflow.com/a/59596600
// Minimal Pure C code to create a window in Cocoa

// $ clang minimal.c -framework Cocoa -o minimal.app

#include <objc/runtime.h>
#include <objc/message.h>
#include <Carbon/Carbon.h>

#define cls objc_getClass
#define sel sel_getUid
#define msg ((id (*)(id, SEL, ...))objc_msgSend)
#define cls_msg ((id (*)(Class, SEL, ...))objc_msgSend)

int main(int argc, char *argv[])
{
    id app = cls_msg(cls("NSApplication"), sel("sharedApplication"));

    msg(app, sel("setActivationPolicy:"), 0);

    struct CGRect frameRect = {0, 0, 512, 256};

    id window = msg(
        cls_msg(cls("NSWindow"), sel("alloc")),
        sel("initWithContentRect:styleMask:backing:defer:"),
        frameRect, 0, 2, false);

    msg(window, sel("makeKeyAndOrderFront:"), nil);
    msg(app, sel("activateIgnoringOtherApps:"), true);
    msg(app, sel("run"));
}

Screenshot of the window in all its glory: