Does Google prefer or penalize reddit, stackoverflow, quora?

I run a technical niche website, and I've been using Semrush for the past week to discover long-tail keywords using some of the techniques taught on BHW.

I've noticed that Reddit, Stackoverflow, SuperUser, or Quora occupy 3 of the top 6 positions for most of these keywords and the difficulty as per Semrush is less than 20 for these KWs.

Is this why Semrush flagged these keywords as long-hanging fruit? I've seen SEO experts say that Google prefers websites to forums.

I am a bit confused as to how these are low-hanging fruit KWs? How would I beat these huge forums?

If this forums occupy like you said 3 from 6, yes, they are Low Hanging Fruits.
The thing is that for that keyword Google ranks forum QA because there is nothing bether to rank.

So if you make good detailed informative and well structured post to answer user intent, Google shold rank you before them.

Yes, there are other factors: DA, domain age, eeat or similar, but the fact that top 3 of 6 are there is a good sign