What does TG E2EE encrypted chat provide? Given its open source clients

What does TG E2EE encrypted chat provide? Given its open source clients

1. Telegram server is not open source.
2. Telegram clients are open source.
3. Users can choose to use E2EE encrypted chat within themselves.
4. User A know User B in real life and they know each other's telegram username,

Q1: With open source clients of telegram, how confidents are we as developers for its E2EE encrypted chat? (so with tech/math, can we prove that e2e enable chats within two users are secure? even we don't know its server code