I'll explain this first: A hard drive doesn't know or care what its contents are. If it gives you sector 123523 when you ask for 123523, it has fulfilled its responsibility.

You don't "install" an MBR, it's just there, in sector zero, because that's where your computer expects it to be by tradition. If sector zero doesn't have valid contents, your computer just won't boot from it. More modern things like GPT -- which can have partitions much larger than a basic DOS-style MBR would support -- have extra data elsewhere as well, at the end of the disk I believe...

You don't get to put partitions in the first 63 sectors just because that's how DOS-style partition tables work, they can't represent numbers any lower than that.

Some disks will come formatted with a sensible MBR. Some just fill it with zeroes. It doesn't matter much, since there's no barrier to doing what you please with it.

The only thing actually stopping you from writing over the MBR is the operating system. When you partition a device in Linux, it breaks it out into separate pretend-devices like /dev/sda1, /dev/sda2, and so forth. Theyr'e safely compartmentalized, they won't let you write beyond their bounds, but it's the OS doing that for your convenience, not the disk itself. You can use /dev/sda itself if you want to talk to the raw device, but almost nothing except a partition editor would need to do so.