Corellium is a low-level iPhone emulator, which is furthermore only provided as a cloud service with Apple's code built-in. They're infringing on Apple's code in so many places that I'm pretty much convinced no lawyers were in the room when they were developing this.

Traditionally emulators are distributed without any infringing code; it's up to the user to extract what they want to emulate from devices they own. For example, 3DS emulators don't ship with a copy of Nintendo's system software, they instead require you to buy a 3DS, hack it, and use some GodMode9 scripts to extract the relevant files. Similarly, one could have developed an iPhone emulator that requires the user buy and jailbreak a phone, and dump the relevant OS files (or extract it from an IPSW available from Apple, etc). This would have most likely been legal.

Projects like Darling are a step removed from even that. Instead of providing a tool that lets the user run Apple's OS in emulation, you instead write your own OS that provides all of the relevant APIs/ABIs necessary to allow software dependent on Apple's OS to run. This tool can then be distributed entirely freely with zero Apple ownership involved. This approach is entirely legal, at least for another week or two, depending on if the Supreme Court understands what an "API" is better than the Federal Circuit.