Hello,
As a continuation of my question:
As I understand it, “it is impossible and one does not intend” is said about a situation where a person, against his will, finds himself in a certain place (“impossible”), and although he encounters the prohibition, he still has the ability not to derive benefit from it (“and one does not intend”). An example I saw: participating in a military ceremony where women are singing.
In such a case, everyone agrees it is permitted.
Our case, in my opinion, is not similar.
As I understand it, your interpretation is: “impossible” — in modern reality, it is impossible without internet; “and one does not intend” — he has the ability to avoid that prohibition, and therefore the conclusion is that it is permitted.
In my opinion this is a mistake, and in this case the correct formulation is: “it is possible and one does intend.” “Possible” — to avoid browsing (difficult, but possible); you are not browsing against your will. “And one does intend” — since it is impossible not to derive benefit from the prohibition, you necessarily do intend it, and therefore the conclusion is that it is forbidden.
As for your example: when a person walks in the street, he can separate himself—he can avoid going to places prone to this, or lower his head; that is, he will not necessarily encounter the prohibition. In contrast, when a person browses the internet, he cannot separate himself, and he also cannot avoid deriving benefit, and therefore the law is that it is forbidden under the rule of “it is possible and one does intend.”

This is a common mistake. See the medieval authorities (Rishonim) on the passage in Pesachim, who explicitly wrote that “not possible” applies even if it requires only a very small effort (and the Chafetz Chaim wrote this as well in his well-known passage on this law).
From this it follows that the category of exemption here is not coercion, but something else. To understand this, think of a situation where you have two routes to your destination, where route A is slightly longer than route B. On route B there is a shop with perfumes used for idolatry, and someone passing there cannot avoid smelling them (and there is a prohibition of deriving benefit from idolatry involved). If you choose route B despite the shop, there is no prohibition in this, because the choice of that route was not made in order to smell, but because it is shorter. Therefore the smell falls under the category of “one does not intend” (you are not going there for that), and so there is no prohibition in it. The exemption comes from the laws of an unintended act, not from the laws of coercion.
Of course, in this example one could still further analyze the category of “since he does derive benefit,” and so on, but this is only an example.

Why does “since he does derive benefit” override the rule of “one does not intend”?

That is a different point, and this is not its place. It is not connected to the discussion.