What it does
Every night, at a time you choose, it posts one short summary of the day into the group: what was discussed, what was decided, and who was driving it. You pick the voice — a factual summary, or a roast with some attitude.
Any time, anyone can type /summary to catch up on the last day, or /summary 3h, /summary 2d for a different window. In a group with topics, it summarizes the topic you are in; add all for everything.
It remembers. /ask answers questions from the whole history — what did we decide about the venue? — and /decisions lists what the group settled.
Setting it up
Quickest: use the KeeperBots bot. Create your account, pick it, and tap the button that adds it to your group — no bot to make, no token to paste, and privacy mode is already off.
You create your own bot with @BotFather, turn its privacy mode off (the step people miss — without it a bot can't read the group), and paste its token into KeeperBots.com. The setup checks each step with Telegram itself, finds your group for you, and never asks you for an ID.
Nothing to install and no server to run. It is hosted for you.
Running a Discord server instead? The same bot does the same job there — you create an app, switch on the Message Content intent, and paste its token. A Discord channel works like a Telegram topic, so per-channel summaries come for free.
What it can't do
Telegram gives bots no message history, so it summarizes what was said after you add it — not the years before.
It can't flood the chat: its own posts are capped at one every ten minutes, and answers when someone talks to it are limited per person, per hour and per day.