You are right. It is not possible for a bot to start a chat with a user after user's joining in public channel. But there is one way to have something like that.

In telegram the only "legit" way to do so is to use "Automated request approval flow". You must have a private channel with enabled "send requests to join" option. Only in that cases bots without being directly activated are allowed to start chats with users(who have sent a request and are waiting approval to join your private channel).

You still can have, for example, user-bot, which technically is allowed to write first(because it is just a user controlled by code), but after you will get some threshold of started chats with "strangers" per day, account will get "Spam ban" and lose ability to start a chat with non-contacts for a limited(or not) time.

So, if you are willing to make a good impression on new users, just do your best at making you channel prettier. 🤓