Ewa is like a greeting.
But ewaja is different it's hard to explain but when a friend did something cool you just say "ewaja" or when a friend is saying something that you agree with you say "ewaja".
You: "Ewa, how are you?"
Friend: "I'm great man, how are you?"
Friend:" I hate the English teacher, she's so dumb."
You:" Ewaja, same."
Friend:" I just skipped my exam but idc."
You:"Ewaja."