Zephyr in Practice
Everything up to here has treated the RTOS as a library. FreeRTOS is a handful of .c files you add to a project you already own: you keep your linker script, your startup code, your CubeMX-generated HAL_Init(), your main(). The kernel schedules; you do everything else. Swapping FreeRTOS for ThreadX would change the function names and almost nothing structural.