Beginner help (obj-c, metal, reading inputs)

Hey all. I'm new to Apple's ecosystem and need some help with a few basics.

Specifically, I've started with Xcode 14's metal game code and am struggling to read user inputs. I've tried variations of the below without luck:

#import "GameViewController.h"
#import "Renderer.h"

@interface InputView : NSView

@end

@implementation InputView

- (BOOL)acceptsFirstResponder{ return YES; }

- (void)mouseDown:(NSEvent*)event
{
    printf("mouse down");
}

- (void)keyDown:(NSEvent *)event
{
    printf("key down");
}

@end

@implementation GameViewController
{
    MTKView *_view;
    
    InputView *_input_view;

    Renderer *_renderer;
}

- (void)viewDidLoad
{
    [super viewDidLoad];

    _view = (MTKView *)self.view;

    _view.device = MTLCreateSystemDefaultDevice();

    if(!_view.device)
    {
        NSLog(@"Metal is not supported on this device");
        self.view = [[NSView alloc] initWithFrame:self.view.frame];
        return;
    }

    _renderer = [[Renderer alloc] initWithMetalKitView:_view];

    [_renderer mtkView:_view drawableSizeWillChange:_view.bounds.size];

    _view.delegate = _renderer;
    
    _input_view = [[InputView alloc] initWithFrame:_view.frame];
    
    BOOL fr = [_input_view becomeFirstResponder];
    printf("fr %d\n", fr);
}

@end

But I also don't understand the basic structure of this boilerplate application. I see my main file is this:

#import <Cocoa/Cocoa.h>

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // Setup code that might create autoreleased objects goes here.
    }
    return NSApplicationMain(argc, argv);
}

It doesn't seem to touch any of the rest of the code (GameViewController, AppDelegate, etc.), and yet it obviously does, since it runs and renders things. I assume some build magic is happening, but where can I find information on that? What official resources can I read to understand how this app is being built? I've been having a lot of trouble navigating Apple's official documentation and could use some direction.