Hello there.

You stumbled over a running instance of the usernameToChatAPI project. The purpose of this API is to provide a way for Telegram Bots which are using the HTTP API to resolve usernames to chat objects. This mitigates the need for these bots to write their own interaction with the foreign MTProto API. Instead, they can use this API and profit from less setup and known objects. The project is Open Source and can be found at https://github.com/Poolitzer/usernameToChatAPI. It is licensed under GNU GPLv3.

If you are interested to integrate this API in your project, amazing. You will need an api key to use this project. Since you discovered this text on the https://usernameToChatAPI.de URL, https://t.me/poolitzer (me \o/) is/am the owner of this instance. I am willing to give you a key, just PM me on Telegram, tell me how many requests per time you are going to do roughly and I will see how it fits in the correct project utilisation.

Once you have the API key, head over to http://usernametochatapi.de/api_doc to check out the (very small) documentation for this API.

The great python-telegram-bot library has a contrib submodule, which provides a neat integration of this API in the library. Check it out at https://github.com/python-telegram-bot/ptbcontrib/tree/main/ptbcontrib/username_to_chat_api.