How does one develop a desktop environment (while not having a one to use)? 🤔

The same way you build the front end of a website from a blank text file.

You write it, test it, write and test it a whole lot more, and eventually it will come to be in existence.

If you’re asking how to write it: good ol’ tty and vim

You use the terminal. Then when you have a simple one that is usable you can build on it. Just like you do with compilers

Well simple you dont need a grapical os to write code. A OS can also be very simplistic eg C64, ZX81 etc. which means you can iterate and keep up with the more complex code. GEOS for example was a graphical OS written on the C64 for the C64. and the base OS of the C64 (named kernal) was written in assembly for the chips the C64 used. This could be done via another computer or by hand. Therefore not a egg chicken problem