You are confusing task priorities with interrupt priorities.
RTOS tasks are created by software and scheduled by software, and run when an interrupt is not running. Which RTOS task runs when is decided by software (the RTOS scheduler). In FreeRTOS the lower the task priority number the lower the task’s priority (which in my humble opinion is logical).
Interrupts will interrupt non-interrupt code, and which interrupt executes when is controlled by hardware. The RTOS software cannot influence how the hardware prioritises interrupts, in fact this is not really an RTOS question, no software can influence how the hardware prioritises interrupt. On ARM Cortex-M architectures (but very few other architectures) the lower the interrupt priority number the higher the interrupt’s priority (which in my humble opinion is illogical).
I think if configUSE_PREEMPTION and configIDLE_SHOULD_YIELD are both 0
then probably only the idle task would run - but its not a normal
configuration and I’ve not checked.
These settings do not control time slicing though, that is controlled by
configUSE_TIME_SLICING.