A walled garden means they control the only means of distribution. If you want an iOS device you have to buy iOS hardware and buy apps/music/videos from their app store. You can't even run other browsers despite theirs currently being inferior. At least they finally opened up their devices to other clouds.
If you sell on their app store, they take a large cut of the profits and your sales are subject to their rules. You have no other option except to not sell to their devices.
When it comes to MacOS, the OS is not very customizeable. They actively try very hard to do prevent you from doing what you want with their software. Windows and Linux are much better in that regard. For example alternate shells and WindowsBlinds on Windows or changing Desktop Environments in Linux.