For Notion Backups, it was a mix of Hacker News and SEO, if my memory serves me well. These days, it is mostly SEO, with some word of mouth/directories (SEO is hard to replicate if you are in a crowded market or don't have any unfair advantages).

I'm on the team "start talking about your product early on, even if you don't have the mvp ready". Don't worry about someone stealing your idea.

If the product can solve someone's problem, it is good enough, even if it won't solve all the problems your potential customers might have.