How to make the bot send a message to the user when the user subscribes to the channel?

Question

Python

I saw an interesting functionality, I wanted to implement something like this, can you tell me which way to dig.
The user receives an invite link, follows it and clicks "apply", after which he instantly receives a message from the bot (although the user has never launched this bot before), while the user does not join any channel.
I do not quite understand how exactly the bot first sends a message to a user who has not launched this bot before.

Answer 1

Telegram bot api does not allow sending messages to users who have not previously written to the bot. This thing uses user bots. When you click on submit an application, some kind of trigger fires, which opens access to the user's ID.