How are Operating Systems "Made"?

Question

Creating an OS seems like a massive project. How would anyone even get started?

For example, when I pop Ubuntu into my drive, how can my computer just run it? (This, I guess, is what I'd really like to know.)

Or, looking at it from another angle, what is the least amount of bytes that could be on a disk and still be "run" as an OS?

(I'm sorry if this is vague. I just have no idea about this subject, so I can't be very specific. I pretend to know a fair amount about how computers work, but I'm utterly clueless about this subject.)

Answer 1

Well, the answer lives in books: Modern Operating Systems - Andrew S. Tanenbaum is a very good one. The cover illustration below.

The simplest yet complete operating system kernel, suitable for learning or just curiosity, is Minix.
Here you can browse the source code.

_{(source: cs.vu.nl)}

Answer 2

Operating Systems is a huge topic, the best thing I can recommend you if you want to go really in depth on how a operating systems are designed and construced it's a good book:

Operating System Concepts