technically, Telegram does support end to end encryption but it's not activated by default, so every discussion is saved in clear text on servers accessible by Russian authorities.
When you activate the encryption, you have to forego basic features like push notifications, it's made ever so slightly less useful to make you not really want it.
Signal on the other hand is fully encrypted by default and run by a non-profit organization. Their code is open source and can be audited.
Telegram's official documentation tells their unpaid volunteer workers to discourage people from using E2EE: https://tsf.telegram.org/manuals/e2ee-simple
I really don't understand them why - as far as I read - why it became the defacto app for CP. It's a great thing for the authorities to have achieved. Bravo.
Here, this should be a lot of useful information as to why telegram is bad and why Signal is good: https://soatok.blog/2024/05/14/its-time-for-furries-to-stop-using-telegram/
telegram is russian. i dont trust russian.
https://www.securemessagingapps.com/
This is a decent resource on some technical differences, however I don't know how up to date it is
There may have been further changes after the arrest last summer
however I don't know how up to date it is.
A changelog is here: https://www.securemessagingapps.com/changelog.html