Text is via mailservers. Attachments (photos, audio files, video files, and other files) are via mailservers. Video calls are peer-to-peer (=directly connected). Audio calls are peer-to-peer.
Peer-to-peer calls reveal your IP address unless you use a proxy.
You can see the text-message public-key fingerprints at [hamburger menu icon]>“Encryption info”. You can manully export public and private keys from the advanced settings (except on Android). Your phone should not allow other apps to access this.
Autocrypt sends your public key with each message, but your private key is only on your devices.
You cannot rotate your key yet. If you make a new profile, you get a new key.
Not yet possible.