Ask HN: How do telegram directory websites find groups/channels?

I wanted to know how directory websites (like telegramchannels.me, telemetr.io, etc.) find channels/groups.

They can't use an "official way" since the discoverability on telegram is really bad.

I don't think they can 100% rely on scraping because many channels are not even on google.

I don't think they can 100% rely on user submissions (i.e. I submit my own channel) because they have a lot of channels/groups for countries where no one knows about those website.

Any ideas?