When signed data is packaged as PKCS#7, what all content PKCS#7 could be there(Ex. certificates, signature, etc.)? And what are the purpose of packaging all those contents?
You can embed any data within a PKCS#7/CMS structure. The Cryptographic Message Syntax (which is the new name for PKCS#7) knows the types EnvelopedData for encrypted data and AuthenticatedData for signed/MACed data.
That way, you can create an enveloping signature, thus storing the signature and the signed data in a single file even when the format of your data does not support it.