Back in the late 2000s and early 2010s many of these large companies billed themselves as open platforms where you can express yourself. Twitter's Jack Dorsey said in 2012 that "We are the free speech wing of the free speech part". Now he says it was joke.
The terms of service on these platforms appears to be enforced selectively, information appears to be selectively censored and a lot of the time it appears to be based on their politics. Also a lot of internet jokes that have been around for years are being censored because the people doing the moderation don't understand the odder internet subcultures.
So It isn't just tech platforms being uncomfortable with certain viewpoints. It a combination of things that make it seem that these companies are censoring viewpoints (some not even that extreme) on one side of the political spectrum. That is what people are complaining about.
> people flock to “community bubbles” like Parler, TheDonald, etc where they feel welcome.
This isn't true. People are removed from platforms and go to another service where they won't be censored or banned and then a bubble forms.
> 3. This marginalisation leads to increasing extremism; going from the likes of “immigrants bad” we saw in 2016 to disputing and attempting to overturn democracy.
This is a strawman of what these people believe. The vast majority of American conservatives and Trump supporters I know don't say "immigrants are bad". They said "Illegal Immigrants are bad". There is a very important difference. The former is clearly xenophobia, the latter is not.
As for "over turning democracy". They see that there was a lot of odd things that seemed to happening upto and including election night. They feel that there has been voter fraud. Whether that is true or not I have no idea and it looks like it will be settled in court. But they believe they are preserving democracy not overturning it.