How the check works
Telegram usernames must be 5 to 32 characters long and may contain only Latin letters, digits and underscores. They cannot start with a digit or underscore and cannot end with an underscore. Bot usernames must also end with “bot”, for example adsun_bot.
The checker opens the public t.me page for the name. If a public user, bot, channel or group uses it, Telegram shows its title — the name is taken. It also reads Fragment, where short and premium usernames are auctioned or sold.
A name that looks free can still be held by a private account, a deleted account in cooldown, or be reserved by Telegram. The final answer always comes from Telegram when you try to set it.