Ok, could you zoom out and take a picture of the entire unit?

Is this wood frame/drywall construction, or something else?

What is it that you want to accomplish / how much are you willing to spend to do it?

Here's the thing: Yes, I think you are correct, they put a replacement window inside the frame of an existing unit.
Clearly this was done to "get in and get out" without disturbing the drywall, etc. and it doesn't look great.

The alternative is taking out the window, removing the metal frame, possibly getting into some drywall, exterior siding, etc.
and getting a new window unit that fits the opening.