Where did your first 100 paying users actually come from?

I'm building a consumer app, and I've read plenty of advice on what should work. I'd rather hear what did.

Here's where we are, to start honestly: paid ads on Meta bring installs but are expensive for a small team. SEO started paying off only this month, the surprise was that the first clicks came from translated pages (Ukraine, India, Poland), not from English. Store search brings a steady trickle. Nothing yet that I'd call a channel.

So, for those who got past this stage:

1) Which single channel brought your first 100 paying users (not signups)?

2) How long did it take before it started working?

3) What did you spend real time on that turned out to be useless?

B2C answers are especially welcome, most advice I find is written for B2B SaaS.