I've notice that certain questions in casual speech end with だ, for example:

何してんだ
本当なんだ
どこに行くんだ

Are these types of questions with だ actually used in real life? If so, who can use it and to whom can be used?

See a translation

何してんだ could be a question . But it depends on the tone . Basically it means "why did you do it?"
But depending on the tone , it means something like "oh my God ! (I don't believe why you did it) .

本当なんだ can't be a question .
It means "That is for real . I got it" or "This is for real . (I am never lying/joking)" . It depends on the tone .

どこに行くんだ could be a question .
Basically it means "where are you/he/she/they going?" .
Depending on the scene , it could mean something like "I am wondering where you/he/she/they are going"

We don't use it to our senior . Depending on the tone , the first one and the third one could sound bossy and all of those could sound kind .

だ is used to assert something as true .

@nanonando25
I am sorry . I am shy .
But if you said it kindly , it probably sounded like kind . So you don't need to be so afraid of , I think .