Did Vatican II undermine the authority of the Church?

Ok, so I was wasting my life on Reddit debating politics with strangers and then I came across a particularly reactionary (you know, pro-monarchy, anti-Enlightenment, anti-democracy, with alt-right and fascist sympathies) Catholic who claimed that Vatican II basically destroyed the Catholic Church because it undermined its authority. A chart showing declining mass attendance also came up. He also claimed that the introduction of "self-defeating" ideology of liberalism undermined traditional Catholic culture. So, naturally I came here looking for answers.

What really happened at Vatican II? Is there any serious opposition to those reforms within the Church today?

P.S. When I say "The Church" I mean "The Catholic Church".