I have Downloaded the Catalina installer from multiple sources app store / fetch full installer method via terminal and I run this pointed at the directory and each time it says certificate Untrusted ?
On what macOS version are you running the command? If your system is not updated, it might not recognize the certificate. The response for a Monterey installer is “Status: signed by a certificate trusted by macOS”
Anyway, that signature doesn’t mean much. Here is a 43MB Monterey installer with a valid signature. You can add random files into it and the signature remains valid.