I personally haven't seen these as I keep my feed limited to tech news and memes but what you can do is click on "report"* And Twitter may take action. I have had bad experience myself with the reporting mechanism as it does not take action always on reported tweets. That I blame on the Twitter execs who do not have moderators who are properly aware of the region of the world they have to moderate. But broken as it is, it still works.

Parler has a report button from what I have heard but it is only for show. And from the former CEO Matze's own words, they allow all constitutionally allowed speech which unfortunately opens the doors for actual Nazis to flood the site with their hatred.