Telegram doesn’t default to encryption. All your messages are stored and can be viewed by anyone with enough privileges on Telegram’s infrastructure.
Telegram’s “secure” 1-1 messages are limited to the point of being useless and not worth using. It’s a dark design pattern created to discourage their use, ensuring you give them all your data.
Private messages on Matrix have been end-to-end encryption (E2EE) by default since 2020 - https://matrix.org/blog/2020/05/06/cross-signing-and-end-to-end-encryption-by-default-is-here/
For anyone considering Telegram for privacy:
TL;DR - Matrix is more private than Telegram.