Some embedded systems have Real-Time Operating Systems (RTOS) and some of them not.
Real-Time Operating Systems is a platform for building applications for embedded systems,
Smaller Software applications do not require RTOS but the medium and large application requires RTOS.
The RTOS is used in embedded systems.
RTOS requires smaller memory as compare to General-Purpose Operating Systems (GPOS) and it provides high performance for real-time embedded devices.
Many small embedded devices do not contain RTOS they just have application code without RTOS.
Every Real-Time Operating Systems have the kernel.