Citadel typically keeps everything consolidated into a single directory hierarchy (which is /usr/local/citadel by default). This is what you will see in an Easy Install, AppImage, or Docker build.
If you have an older installation and installed the legacy .DEB or .RPM packages, you will need to migrate away from the old LFHS layout.
/usr/local/citadel Citadel Server, utilities, and the command-line client /usr/local/citadel/data All databases live here /usr/local/citadel/keys SSL keys and certificates /usr/local/webcit WebCit web user interface for Citadel /usr/local/ctdlsupport Libraries such as libcitadel