How do you folks get notified of comments on your post of Hacker News?

I just hit the "threads" link in the menu bar at the top.

Otherwise, I don't think there is a built-in notification mechanism.

Me too. And it's a bug, not a feature, that there's no explicit notification mechanism. When other posters post in bad-faith, go strawman or ad-hominem, that may get flagged or the entire thread may get downvoted; which in turn gives people disincentives for bad behavior.

They don't have a built-in notification mechanism; if you missed replying to a comment, you can't even reply to them after a few days. I had posted about selling my project https://techinterviewexp.site/ but I missed the comments and now I can't reply to them.

HN Replies (https://hnreplies.com/) will tell you about replies to your comments but I don't know of anything that tells you about comments on your posts. I just check the Submitted link (https://news.ycombinator.com/submitted?id=yourusername) every once in a while.

Yeah, I also started doing the same.

Because of this, I missed some nice comments on a post I had mentioned about selling my project https://techinterviewexp.site/ And, now I can't reply to them.

I don't.