Since you installed Caddy using a package manager and are running Caddy as a systemd service, it’s running as the caddy user, whose HOME is /var/lib/caddy. So the root cert is in Caddy’s storage at /var/lib/caddy/.local/share/caddy/pki/authorities/local
You’ll need to copy and install the root cert to whatever machines/browsers need to trust connections to Caddy.