Apple makes you sign the same contract that e.g. google makes you sign, which says basically "I sign away all rights to anything I create to the maximum extent legally permissible under California law" (You can look up what those exceptions). For companies like Apple, this basically means they own everything you might do, since their "area of business" in essentially limitless.
However, once I signed away my rights, the experience at Google and Apple was quite different. At Apple, I waited months, with multiple follow up pings, to get approval from a lawyer for a one line trivial patch for an OSS project. I had to give an argument that my contribution provided a direct business benefit to Apple, and generating goodwill in the community was explicitly listed as a reason that is not valid. I couldn't contribute to any Google run OSS projects either (some issue with the CLA, not sure of the blame, TBH).
In contrast, at Google you are encouraged to contribute, don't need any approval for normal OSS projects, and I have easily gotten approval to release two personal projects.
And at Microsoft we didn’t need any approval at all for things done in our own time and own equipment - provided it wasn’t competing with anything the company was working on.
It gets better: during the Windows 8 launch and for the life of Windows Phone 8/10 we were actively encouraged to build own apps for their respective App Stores (cynically this was in-part to boost app count numbers, but also to make us motivated to dogfood the platform, provide feedback, etc). IIRC we were expressly permitted today use company hardware too - just not during core business hours. That said, I openly worked on my never-released clone of “Lose Your Marbles” during my downtime in our teamroom office during the day - right in front of our team’s Partner-level director who didn’t bat an eyelid...
From my friends that work there - the answer is "it's complicated". Some of them do get away with it, obviously such as those that work on open-source software at Apple (like Safari, Darwin, etc) - or those with non-computer-related side-projects like photography or artisanal furniture making...
Apple seems to have inherited Microsoft's previous institutional paranoia about open-source software: the legal dept is concerned that if an employee casually browses GitHub and is inspired by some GPLv3 code that they could rewrite themselves for use in a proprietary Apple product then the lawyers consider that Apple product now possibly tainted (as rewriting code or being "inspired" by other code still legally counts as a derivative work, even if absolutely nothing was copy and pasted).
Microsoft lost that attitude around the same time Satya became CEO - I was at Microsoft when the transition happened and it was a refreshing change of policy that really came top-down. Of course we still had annual SBC training to remind us to always verify the licenses of any third-party or open-source components we use (and GPL is generally a big no-no, of course, without special dispensation from LCA) but the idea that a product's legal status could be tainted by casual browsing went by the wayside. I think a lot of the change came from a wave of sanity at the C-level when they realised the company was not actually being actively destroyed by meritorious - and meritless - open-source copyright violation lawsuits, and the few incidents that did occur (like the Windows ISO-to-USB tool debacle) were overblown with minimal impact to the company.
But Apple's paranoia isn't just about legal matters, but also out of concern that if Apple-watchers know who works for Apple and monitor their GitHub accounts then they'd be able to see which areas of technology interest those people, which may in-turn drop-hints about what Apple is working on (e.g. if we suddenly see a bunch of Apple folks Starring and forking LiDAR libraries for private use then that hints Apple is working on a LiDAR product... which they announced this week that they are: https://www.theverge.com/2020/3/18/21185959/ipad-pro-lidar-s...
Now, as someone who believes they'd otherwise make a great contribution to Apple as a valuable SE asset (yes, I'm being self-aggrandizing) this policy of theirs is a huge deal-killer for me. Not just because I own some popular GitHub repos with GPL licenses, but because I also have popular (and profitable) side-projects that only use a few hours of my time each month - and Apple is just as paranoid about those things as open-source projects are as vectors for leaking company movements, even unintentionally.
Heh - I remember shortly after I did NEO at MSFT and filled-out the "prior inventions" paperwork for my side-projects, my team lead admonished me for wasting his time looking at my dumb Google AdWords-supported websites - though he did agree the definition of "prior invention" was too vague.
(Footnote: if you're a recruiter or hiring-manager at Apple and you're reading this, and you agree that my side activities won't be a problem, please reply to this comment - I'd love to work on WebKit or Darwin at Apple :D!)
