dyld VS FreeRTOS-Kernel

None of what? In case I understood you by chance, there's a whole darwin stack on github, grouped neatly at https://opensource.apple.com/releases/. There is a lot of argument about what's open source and what's not, I consider permissive licenses as open source. I did never claim that any Apple software is open source, but due to OS's modular nature I'm willing to say that macOS and iOS consequently is largely open source

A lot of Apple software including the operating systems is open source, though in practice many of the key components that you might need to solve a problem or understand a bug are missing from these repositories.

Here is the port - https://github.com/FreeRTOS/FreeRTOS-Kernel/tree/main/portable/ThirdParty/GCC/Posix

Freertos has support for generic cortex m3: https://github.com/FreeRTOS/FreeRTOS-Kernel/tree/main/portable/GCC/ARM_CM3 It could very well work almost out of the box with your mcu.

the only OS written in iso c i found was RTOS and they have open bug reports due to strict aliasing in simple things like their linked list implementation, which they don't bother to fix because somehow that causes even more problems (?!): https://github.com/FreeRTOS/FreeRTOS-Kernel/pull/54

You likely put the requirements of POSIX just to have an excuse. Fyi, FreeRTOS kernel, the only thing you need, is here: https://github.com/FreeRTOS/FreeRTOS-Kernel/

- https://github.com/FreeRTOS/FreeRTOS-Kernel (architecture, C)

FreeRTOS comes with 5 quite simple reference implementations of the heap memory allocator. You can study how it works.

The main branch is single-core. You're looking for the SMP branch.

On non-POSIX systems, malloc & free are implemented differently. EG FreeRTOS has some very simple malloc (and optionally free) implementations, eg heap_2.c (has malloc & free, but doesn't consolidate blocks), or heap_4.c (more complex, tries to prevent fragmentation by consolidating blocks, still less optmized & simpler than the glibc malloc/free used by most Linux systems).