What is the aTalk (Jabber / XMPP) application, how does it work?

According to the introduction of cmeng, aTalk (Jabber / XMPP) is a Communication app on the Android platform. There is currently a 2.2.2 version released on Mar 29, 2020, you can see the details below.

aTalk - xmpp client for android rich in features:
* Instant messaging in plain text and End-to-End encryption with OMEMO or [OTR
* SSL Certificate authentication, DNSSEC and DANE Security implementation for enhanced secure Connection Establishment
* OMEMO encryption in group chat session enhancing privacy and security
* OMEMO Media File Sharing for all files including Stickers, Bitmoji and Emoji rich contents
* Support http file upload for file sharing with offline contact and in group chat
* Support Stickers, Bitmoji and Emoji rich content sharing via Google Gboard
* Send and receive files for all document types and images with thumbnail preview and gif animation
* Auto accept file transfer with max file size option
* Implement fault-tolerance file transfer to ease and enhance file sharing reliability
* Enhance and harmonize UI for file sharing in chat and chatRoom
* Support share, quote and forward of messages and medias with previews prior to sending
* Support multi-instances of audio media player with functions: Play, Pause, Resume, Stop and Seek
* Support unread message badges in contact and chatRoom list views
* User selectable option for heads-up notification and quite hours
* Share of social media links are tagged with thumbnail and title
* XEP-0012: Last Activity time associated with contacts
* XEP-0048: Bookmarks for conference room and autoJoin on login
* XEP-0070: Verifying HTTP Requests via XMPP entity for user authentication without password entry
* XEP-0071: XHTML-IM Support chat messages containing lightweight text markup
* XEP-0085: Chat State Notifications
* XEP-0124: Bidirectional-streams Over Synchronous HTTP (BOSH) with Proxy support
* XEP-0178: Use of SASL EXTERNAL with TLS Certificates for client authentication
* XEP-0184: Message Delivery Receipts with user enable/disable option
* Implement Jabber VoIP-PBX gateway Telephony, allowing PBX phone call via service gateway
* Join or create room with full room configuration support for owner
* Integrated captcha protected room user interface with retry on failure
* Support both voice and video call with ZRTP, SDES and DTLS SRTP encryption modes
* ZRTP crypto algorithms uses SHA-2 384, 256bit ciphers AES256 & TWOFISH-256, enabling elliptic with Curve 25519
* SDES encryption with AES256 and AES192, acceleration using OpenSSL
* Support simultaneous media call and message chat sessions
* Unique GPS-Location implementation as standalone tool, send locations to your desired buddy for real-time tracking or playback animation
* A 360° street view of your current location use for self-guided tour. The street view tracks and follows your direction of sight
* Built-in demo for GPS-Location features
* Integrated photo editor with zooming and cropping, user can update the avatar with ease
* Last message correction, message carbons and offline messages (OMEMO)
* Stream Management and mobile network ping interval self-tune optimization support
* In-Band Registration with captcha option support
* Multiple accounts creation
* User selectable Themes support
* Multi-language UI support (Bahasa Indonesia, English, German, Russian, and Spanish)