Help clarifying which signatures needed during attestation signing

Full disclosure… I’ve always clicked as many boxes as it’ll let me for the $ARCH$ I’ve built. Now I’m looking to use the API to automate signing and thinking I’m overdue for a more thorough understanding of which signatures are truly required. Does anyone have a brief summary they could share? Are the signatures truly 1:1? Can you target only the oldest OS code and new OSs “just work”?