For my side hustle I have to ramp-up on other areas like marketing, legal, sales, ...
So I wonder if there are similar high-quality sources like HN for these areas.
Other industries just don't have an HN equivalent, either for lack of trying or because hackers are good making and using things like HN when others aren't.
Dev.to and Zenn for longer-form technical writing, though the quality varies a lot.
For architecture and system design specifically, the Software Architecture subreddit (r/softwarearchitecture) has surprisingly good discussions.
HN remains the best for the intersection of tech + business + ideas.
If you're interested in programming then r/programming
If UFC, for example, then
r/ufc
r/mma
r/python
Just whatever it is you're interested in, there's likely a subreddit for it. The more niche it is, the better the quality of the sub.