I've got a few dozen patents. Been through lots of litigation. Even people who think they understand them often don't and rarely will even two patent attorneys agree on anything. Patents also aren't cheap, and good ones keep costing money.
Last I checked, Edelbrock didn't make iron heads and alum heads are a completely different design due to strength and material property differences. Those differences are mostly internal, but enough to not be a direct copy. Being compatible with bolt on stuff isn't the end all when it comes to IP.
Sometimes patents protect a design, sometimes it's a specific architecture, sometimes it's a process. It all depends on the art being claimed.
The purpose of a patent is to teach the public the art in exchange for the sole right to produce or license that art for a period of time (usually 20 years).
Improvements can be patented also, but are prosecuted differently and having a patent on an improvement doesn't give one license to the original art.
Trademark is an entirely different even if related matter.
Even when the patent is expired and it's legal to copy it, personally I still find it distasteful. There's always a way to improve any product which makes a direct copy a super lazy effort. Worse, most knock offs are a step backwards in terms of quality.