First the operating system used by early Apples: I merely gave it the name - it was called Borland at that time. I do not know why, but it was. Whether Apple used it with full rights I know not either. But I am correct in stating that the system was very well received and the early machines sold out consistently.
As for a company using someone else's system, I seem to recall Mr Gates buying DOS on the cheap when he was pitching to IBM - his mum being on the IBM Board of Directors for that pitch being a very nice side story.
My point about 'my life's work' is that I was talking about my own computers and, as these are the tools of my trade, I am picky, very picky. I live or die commercially on the basis of reliability and security. My machines are chosen accordingly. I am not using machines provided by my 'employer', I am the employer.
The unique Operating System on my main workstation is just that, unique. It was originally written as an AI shell for a project and adapted. It uses LISP. I have no idea how it works but the man who set it up was/is an AI specialist with a healthy contempt for all things ordinary. For obvious reasons - well, obvious if you know LISP - I cannot run the system on an ordinary machine. So, my Macs (2) and PCs (2) are used for different tasks entirely.
I will add that the work that went into the OS was impressive at a very high level. The project was designed to, among other things, create a system that would run on about one tenth the code of anything else out there - hence the use of LISP. It was completed in about 1990. I have had programs written for it and was lucky in that a couple of very big commercial ones would run on it - again, don't ask me how, but I know it mimics DOS part of the time and hitches a ride on a windows-like interface that came with the original Xerox Ventura. I do not launch missiles with it, I go from writing to publication of notes and books. It has three main advantages: speed, security and, I can produce materials that are distinctive - it does things that normal publishing programs cannot. Obviously, my work hardly plumbs the depths of the OS. But that is irrelevant. I was given the opportunity to use it and have not regretted that for one moment.
As you say you are a developer, I would expect you to at least recognise the difficulty of creating a complete OS. Several orders of magnitude indeed.