KeePassX is an open source c++ application. It was one of the cross platform applications to manage KeePass databases. It has not been under very active development for a while.
KeePassXC is a fork of KeePassX which is under active developement. They have added many features and improvements and has stayed up to date with kdbx updates.
It hasn't: https://keepassxc.org/docs/#faq-audit
Thr original Keepass was at first a Windows only app, so KeepassX was created as a cross platform reimplementation using Qt. Nowadays Keepass uses .Net and also is cross platform but Linux users prefer Qt because it feels more "native" than .Net.
KeepassXC is a more recent fork of KeepassX, which added a bunch of extra features. Notably, it added support for version 4 of the keepass database format, and it also allows you to use a browser extension to enter passwords inside web browser forms without needing to cut and paste.