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.
I built a HN clone for someone who wanted it focused on just E-Commerce discussions, but it failed to take off. Also didn't help that the person wanted to monetize it using a pay-to-use model. It never took off.
But it's impossible without their equivalent of @dang managing their equivalent of a forum of smart people who have a deep allergy to being marketed to/advertised at/BS'd or enshitified at. They need a HN-like immune system, often grossly overzealous and way too self-serious, that actively polices things like this.
That's why there's no HN equivalent elsewhere.
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.
The Fediverse (specifically Lemmy and PieFed) have much higher signal to noise ratios and are frequented by the same folks you see here in a lot of cases. Additionally, their APIs are open and free.
In the meantime I've bookmarked quite a few more on literature, politics, and such, if it's of any interest. I've also made (a fairly weak) case for Jacobin here: https://news.ycombinator.com/item?id=44309069
You decide how to interpret this.
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.