Correction: "Nah, I have just arrived".
It may help to realize that only is related to the word one (you can think of it as the "adverb" form of one).
Just has several meanings, one of them is used to express that almost no time has passed between an action and now. You can't substitute only when this meaning is used.
I just got home. Let me take my coat off and then we'll talk
If you really want to use only you have to include a time expression.
I only got home a second ago. Let me take my coat off and then we'll talk.
Just can also be used to express that you don't want or don't want to do anything except X.
I just/only want to go home = I don't want to do anything else except go home.
I just/only want some candy = I don't want anything else except candy.
Just however might be more typically used if:
- someone is threatening you and you're saying you want to do something to escape, or
- you are being accused of something and are trying to explain you want to do something else instead.
I just want to go home (you're scared)
I was just trying to look at it (someone's accusing you of stealing)
Just is definitely a weird word in English. Google lists 6 definitions and they all mean very different things. The meaning of only is only one possible meaning of just.