When I tried Signal, I saw an empty list and had to add numbers / find people myself. Not much in the way of onboarding IMO. My complaints are likely met with "duh, that's how it works, idiot", I believe that's just how Signal is designed as it's a privacy-centric app.
I'm just looking for a non-Facebook WhatsApp alternative, rather than a complete focus on privacy and E2E messaging, so I don't think Signal is "bad" but it's not what I'm looking for. I just want Facebook out of my life and its grubby hands off my data.
There were some permissions settings that I didn't pay close attention to, but maybe you didn't allow the app to control your calls or see your contacts on setup?
That's not how it works; Signal directly gives you the list of contacts that are available, just like Whatsapp or Telegram.
The most likely is that you had an empty list because no-one in your contacts have it.
Here in the UK WhatsApp and messenger are the only ones that most people I know are using.