Websites, mobile apps, desktop apps and mobile OSes are developed and updated using the desktop OSes, which I would call the 'master OS'. But who updates the 'master'? How do the devs upgrade Windows 10 to Windows 11 using Windows 10?

I have some experience in computing but software development for operating systems is completely mysterious for me. I have had this question ever since I learned about software development in general.

I saw Apple say how they use the Macs to build all of their other products and softwares, but they never answer how they build macOS itself. I understand how these companies could design an upgraded or a brand new computer by designing its new architecture as well as the circuitry and the components underneath with the help of a program like CAD. What I don't understand is how they upgrade their existing software they themselves work in, especially when it has completely new features the old one doesn't have. I feel like this is similar to a person performing a brain surgery on himself.

I would really appreciate if someone could ELI5 but only dumb it down enough for a person that understands how to really work with computers and knows the general theory of programming , like an amateur or the family IT guy.