I think Trump is a large reason why the Democrat party has moved so far to the left. There are positions today taken by mainstream Democrats that would be considered radical even for fringe groups ten or twenty years ago.
I'm not sure. I think they are both symptoms of the same underlying issue. The centrist consensus of the last few decades resulted in mainstream political parties being almost indistinguishable from one another. This led to a widespread feeling of disenfranchisement amongst the general public, millions of people who felt that no-one represented them, and that it didn't matter which party won, the result for their own everyday lives was always the same.
There are people to the right of centre who feel unrepresented, and people to the left of centre who feel exactly the same. The rise in the US of politicians such as Trump and Sanders is mirrored in other nations. Here in the UK, we had Jeremy Corbyn on the left, branded an extremist by his own party, and the pro-Brexit faction on the right, previously extremists within their own party, but now running the country... sorry, 'running' the country.
Clinton's scandals forced him to move to the center (after initially appealing to the left base), and absent this moderation, he likely would have been removed from office. The W Bush years saw the US move incrementally to the right, but had Kerry won in 2000, the country would not have been run all that differently. You might argue that Obama, who most reasonable people believe hates the US, allowed the Democrat party to move to the left, not only by governing from the left but also by putting up with extremists in his own party.
Trump is arguably to the left of W Bush and is probably to the left of how Clinton governed in his second term on some issues, although is to the right of Clinton overall. Trump was also not very far to the right of where Hilary Clinton would have governed on most issues, although Clinton would have been very corrupt.
If you are on the far left or the far right, chances are that you are not going to vote for a politician in the other party. It is the moderates that are willing to cross party lines and the independents who are willing to vote for different parties. This fact forces national candidates to moderate their platforms. I believe Biden won because he successfully displayed himself as being moderate, and someone who would not bully his opponents the same way that Trump did, obviously once Biden got into office, both of these proved themselves to be false.
I think part of the problem with Trump was that he never made a serious long-term effort to reach out to the other side of the aisle, he only tried to keep his base happy. This was the mistake that Clinton initially made, but corrected himself.