The venues where competitors already work are a ready-made map of the niche. Someone spent time finding them and the result is visible from outside.
One: search by name
Competitor names in search return their channels, and through them the linked discussion groups. The fastest route and the most obvious.
Two: through their commenters
Find the accounts commenting on a competitor's behalf and look at where else they write. One such account exposes their entire venue list at once — usually dozens.
Three: Telegram recommendations
Channels carry a «similar» block. Starting from one competitor, a few hops collect most of the niche: recommendations are built on audience overlap, which is exactly what you want.
Four: mentions
Searching messages for a competitor's brand name shows where it gets discussed. Often those are chats that appear on no list — the places where the audience compares options.
Why do this before launching
To see whether the niche is burnt. If every discussion already holds five identical pitches, yours will be the sixth and perform accordingly. An empty discussion under a live channel is the opposite signal.
And to read what they write: an hour spent on competitors' comments teaches more than a week of your own experiments. Assessing a niche as a whole is in the niche piece.