The data is still on Telegram's servers, unencrypted. They may be doing ads in a channel/group, but I don't think for one second that's where they're going to stop now that they've entered that rabbit hole. By the time you'd realise, it'd be too late, since they've got your data on their servers, unencrypted. This is strictly worse than WhatsApp.

https://gizmodo.com/the-dangers-of-techs-privacy-promises-18...