Generally speaking for any piece, the more friendly pawns in its way, the worse it is.
If a piece can move to where fewer friendly pawns are not blocking its lines, or if friendly pawns can be moved off of those lines, then the piece can be improved. So temporarily being blocked is not something to worry about,
Other general considerations are centralization and being in contact with targets (like weak pawns) or points in infiltration (like on the 7th rank or near the enemy king).
The more pawns that are captured (of both colors), especially center pawns, the more open the position is. But the open-ness of a position is a fairly superficial way to judge the value of minor piece trades.
GMs may mention it casually in a book for beginners e.g. "this knight is good because the position is closed" but in reality each position is unique and it's more about the scope of that piece (number of safe square it can move to) and its proximity to useful squares (the center, targets, or infiltration).
If you want to get really specific, it's not even about the pieces coming off. It's about judging who has the better set of pieces left on the board. E.g. you may trade a fantastic bishop for a mediocre knight,.. if the pieces left on the board favor you as a result.
I'm currently studying minor pieces and I know in general knights thrive in closed positions while bishops in open ones but sometimes I'm not sure if the position would be considered open or closed? Is there semi-open positions or semi-closed ones? If so how would you tell which minor pieces thrive? What minor piece exchanges should you opt for or not go for.