macOS Editing variable string from another class

[abcdefg12345]

I'm trying to figure out how to pass variables between classes correctly and i managed to import variables from other classes but i only edit them in the main class, anyone knows how i can edit the variables on other classes

heres what i did

Main Class.h

#import <Foundation/Foundation.h>
#import <Cocoa/Cocoa.h>

@interface MySingletonCenter : NSObject {
        
@private
    double operand;
    NSString *StringToPass1;
    NSString *StringToPass2;
    NSString *StringToPass3;

}
@property double operand;
@property(readonly) NSString * StringToPass1;
@property(readonly) NSString * StringToPass2;
@property(readonly) NSString * StringToPass3;

@property (weak) IBOutlet NSTextField *Field1;
@property (weak) IBOutlet NSTextField *Field2;
@property (weak) IBOutlet NSTextField *Field3;

- (IBAction)Button:(id)sender;

@end

Main class.m

#import "MySingletonCenter.h"

@implementation MySingletonCenter

@synthesize operand;
@synthesize StringToPass1;
@synthesize StringToPass2;
@synthesize StringToPass3;

- (IBAction)Button:(id)sender{
    StringToPass1 = [NSString stringWithFormat:@"%@", _Field1.stringValue];
    StringToPass2 = [NSString stringWithFormat:@"%@", _Field2.stringValue];
    StringToPass3 = [NSString stringWithFormat:@"%@", _Field3.stringValue];
}

@end;

different class to import the string variable to

string.h

#import <Foundation/Foundation.h>
#import <Cocoa/Cocoa.h>
#import "MySingletonCenter.h"

@interface string : NSObject {
        
@private
    IBOutlet MySingletonCenter *brain;
}
@property (weak) IBOutlet NSTextField *Field1;
@property (weak) IBOutlet NSTextField *Field2;
@property (weak) IBOutlet NSTextField *Field3;

- (IBAction)Button:(id)sender;

@end

string.m

#import "string.h"
#import "MySingletonCenter.h"

@interface MySingletonCenter()
@property(readonly) MySingletonCenter *brain;
@end

@implementation string

- (MySingletonCenter *)brain {
    if (!brain) {
        brain = [[MySingletonCenter alloc] init];
    }
    return brain;
}

- (IBAction)Button:(id)sender{
    [_Field1 setStringValue:self.brain.StringToPass1];
    [_Field2 setStringValue:self.brain.StringToPass2];
    [_Field3 setStringValue:self.brain.StringToPass3];
}

@end