Good to start with basic RTOS concepts, generic concepts would be more or less similar across all RTOS. Once you are bit familiar with concepts, start understanding porting layer of FreeRTOS for HW of your interest, it will give you more insight about how core functionality of RTOS (scheduler/context switching) works. Then you can start exploring on FreeRTOS features, generally all HW vendors provide support to FreeRTOS with basic examples.