When something is broken
Feedback with a screenshot, a reply in Telegram, and separately “Suggest an idea”.

Where: More → Feedback.
One field and a Screenshot button. The reply comes back to this window and to Telegram — to the number you signed in with. The thread is kept, so you can come back to it later: the window opens on the last fifty messages, and everything before that is one Show earlier messages button above the conversation.
Separately there is Suggest an idea: what you need from the builder that is not there yet. What gets asked for most makes it into the roadmap — which is public, showing what is done, in progress or under discussion.
Before writing, it is worth checking this handbook: the usual “it does not work” turns out to be a screen with no way in, a forgotten Update publication, or a list card missing “Remember the tapped row”.