Account
Sign-in, optional Telegram linkage, and the difference between the two.
How you sign in
Signal uses Google sign-in. Your Google identity is used to authenticate you to the Signal backend; it is not broadcast to other participants.
Link Telegram (optional)
Linking a Telegram account is a separate, optional step. Linking Telegram does not sign you in. It lets you identify yourself to a specific Telegram contact you already have, through a secure callback.
Sign-in vs. linked identity
Sign-in is how Signal knows it is you. Linked identity is how you prove who you are to a specific other participant. Signal never uses your Telegram login to sign you in — the two stay separate.
Account basics
You can stop a session at any time. There is no "online" indicator, no "last active" indicator, and no way for another participant to see whether you have signed in beyond whether your current session is active.