Prior to the internet I did know a few people that had their opinions pretty well locked in and wouldn't consider anything else. The internet exposes us to more of those people because, just like in person, they are the ones most likely to keep sharing their opinion; so they are the ones most heard from. And the loudest people always seem like the most people.
That said, I think the internet moves a lot of people from having slight opinions on things to strong opinions on them. When people just interacted in real life they met a variety of people with a variety of opinions and reasons for those opinions. They might meet a few others that share their leanings on something or even join a club with a dozen or so in it. But it wasn't an entire support group of hundreds or even thousands telling them 'you are right, those other people are wrong'. And those groups weren't instantly accessible 24/7. Instead of talking to a co-worker about a wide variety of things they can just visit the group that supports their thinking.
Not only do people now spend most of their time only with like-minded people but even when not on forums the algorithms of search engines and video platforms keep feeding them results that are in line with what they think because the companies know that will keep them looking longer and they will see more advertisements. So information backing the other point of view is out there, but they won't see it because the companies that make all their money off ads are afraid showing them that might make them leave rather than spiraling down a rabbit hole of things that will anger them.
Some people have always been into politics but 30 years ago I don't remember it continually coming up in conversations about completely non-political things. I worked with a guy that was very opinionated and outspoken but his subjects were things that were personal to him. As the internet took off he switched to pretty much anything and everything being able to spin to "That's like <insert politician> they think...."
The early days of talk radio gave people a few hours of what the internet now does, but the huge growth in the number of those types of programs actually follows the growth of the internet. There was a time when the radio talk show people would talk about TV programs, movies and since more of them were local, local happenings. Now the ones that I've listened to off and on for decades have gone syndicated and are mostly politics.
I think, not only has the internet siloed people into groups with similar opinions it has made many from those groups seek out other information sources that reinforce those opinions. A few years back, large newspaper was struggling, like so many, and then it picked a political side and really focused their coverage on how bad the other side was. When they did that, their online subscriber level skyrocketed. When the customers went from getting a variety of news stories, likely with a leaning towards one political side to a large number of stories focused on just one side they were so much happier. Likely because most people aren't accustomed to having their opinions challenged by what they read and hear, even slightly, anymore.
