Reddit mentions of uC/OS-III, The Real-Time Kernel, or a High Performance, Scalable, ROMable, Preemptive, Multitasking Kernel for Microprocessors, Microcontrollers & DSPs (Board NOT Included)

ARM is the best selling microprocessor in the world and has been for a long time. If I had to scavenge one, I could find dozens in my house alone. Simple embedded OS for ARM are easy to find (like in this book: https://www.amazon.com/Performance-Preemptive-Multitasking-Microprocessors-Microcontrollers/dp/0982337531/ref=pd_lpo_sbs_14_t_1?_encoding=UTF8&psc=1&refRID=J1YW00QQ6BKNPKY9WJJ1 ). I guess it would be easier to fab a Z80, but the giant amount of knowledge required to get to that point seems daunting to the point of impossible to me - I wouldn't begin to know how to recreate the conditions to grow a monocrystalline silicon ingot, the diamond saw blade to cut the crystal, the lithography gels, mask films, etc required to make even a Z80). I would much rather try to find another abandoned smart fridge or phone and pull the circuit board and repurpose it.