Where is the core of Unix?

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http://unix.stackexchange.com – I know there are a lot of questions like this, but I certain this one is rare. I am a computer programmer who wants to take on the challenge of building an operating system. I want to base it on Unix because I know that what I make could never compare. Where can I find Unix? I don't mean things like Mint (that's Linux, I know). I mean the thing they are all based on. Where can I get the main files, like the boot loader and the kernel? (HowTos)