Yes, they are very similar in meaning! Both mean that you are relaxing or taking it easy. The difference is subtle: 1. "I'm just chilling" – This is the more common and casual way of saying you're relaxing. 2. "I'm just chilling out" – This adds a slight emphasis, often implying you're unwinding after stress or activity. Both are perfectly natural and interchangeable in most situations.

"I'm just chilling" means you are relaxing. "I'm just chilling out" could also mean you are relaxing, but it would usually mean you are trying to calm down. ( Maybe something made you angry, and now you need to chill out)

Yes, you can use both of these.

I agree with Albert and Camilla.