learning rtos
Yes and no.
Many of the APIs can be similar (start timer, send a message, get a semaphore) but the major difference between an RTOS and a non-real-time OS is the scheduler behavior & implementation.
Things like priority inheritance, interrupt latency, preemption, scheduling strategy and other mechanisms give the RTOS a more deterministic behavior in a real-time setting.
At the application level, if you abstract out enough, most OSs can be thought of as "similar" in that they allow multiple tasks/processes/threads to run concurrently and communicate with one another. But as they say, "the devil is in the details".