> Didn't a court already rule that API cannot be patented. I believe it was a Google case. As long as no implementation was stolen, the API itself is not able to be copyrighted.
That is... not accurate in the slightest.
Oracle v Google was not about patentability. Software patentability is its own separate minefield, since anyone who looks at the general tenor of SCOTUS cases on the issue should be able to figure out that SCOTUS is at best highly skeptical of software patents, even if it hasn't made any direct ruling on the topic. (Mostly this is a matter of them being able to tell what they don't like but not what they do like, I think). But I've had a patent attorney straight-out tell me that in absence of better guidance, they're just pretending the most recent relevant ruling (which held that X-on-a-computer isn't patentable) doesn't exist. In any case, a patent on software APIs (as opposed to software as a whole) would very clearly fall under the "what are you on, this isn't patentable" category of patentability.
The case was about the copyrightability of software APIs. Except if you read the decision itself, SCOTUS doesn't actually answer the question [1]. Instead, it focuses on whether or not Google's use of the Java APIs were fair use. Fair use is a dangerous thing to rely on for legal precedent, since there's no "automatic" fair use category, but instead a balancing test ostensibly of four factors but practically of one factor: does it hurt the original copyright owner's profits [2].
There's an older decision which held that the "structure, sequence, and organization" of code is copyrightable independent of the larger work of software, which is generally interpreted as saying that software APIs are copyrightable. At the same time, however, it's widespread practice in the industry to assume that "clean room" development of an API doesn't violate any copyright. The SCOTUS decision in Google v Oracle was widely interpreted as endorsing this interpretation of the law.
[1] There's a sentence or two that suggests to me there was previously a section on copyrightability that had been ripped out of the opinion.
[2] See also the more recent Andy Warhol SCOTUS decision which, I kid you not, says that you have to analyze this to figure out whether or not a use is "transformative". Which kind of implicitly overturns Google v Oracle if you think about it, but is unlikely to in practice.