Earlier quoted context omitted.
Telegram is unencrypted by default. All standard messages are stored on the server. Telegrams secret chat mode (end-to-end encryption) uses home made cryptography, and has been panned by experts in past. All group chat is in the clear and stored on the server. This is not the case with imessage. Comparing Telegram to iMessage, telegram is not in the same league as Apple. I don't trust either from TLA's or well funded…
It‘s not homemade crypto. It‘s just not the latest and greatest modern crypto but it has no glaring weakness.
So if you are using it on desktop, all your messages belong to Telegram, and whomever they are sharing it with.