My question is: how to join my telegram bot to a telegram public channel that I am not administrator of it, and without asking the channel\'s admin to add my bot to the channel
Another way of "scraping the data" from several channels would be to listen to these channels with a Junction Bot aka Channels, Multi Feed Edge Bot, or alike. You just need to go to the channel info and share the link of the desired channel with the Junction Bot to automate the process of receiving new messages from the(se) channel(s). There are also other quite advanced ways described in their FAQ.
Multi Feed Edge Bot also seems to work O'kay, however, you need to follow these steps to add the redirection:
/add @TelespaceNews https://t.me/joinchat/AAAAAFIaTHjZp8ltykgwEQ