cross posted from the other thread because this is the correct answer, and you’re not really gonna get a better one:
When joining the Apple Developer Program, you get access to a lot of developer resources, including the iOS app icon packs, which include all of the iOS icons for just about all system apps and widgets, etc. pretty much everything. You can use them in your own apps that you develop for iOS and whatnot, but there are some limitations, and they're spelled out in the Developer Agreement that you sign when you sign up to the program.
I haven't been part of the developer program for a few years, so I'm not familiar with the latest guidelines, but they're not difficult to look up on apple's dev website at developer.apple.com
anything made by a 3rd party is obviously owned by them.
So the answer is yes. When you join the Apple developer program, you become a licensee, and you can use them.
Edit: IIRC there are two tiers for the developer program: a free student tier, which allows you access to download beta for any of Apple‘s operating systems as well as any assets, and a $100 paid tier which allows you to actually publish anything you create to the App Store. so you can get a licensed access to all of the assets for free. But, again, how and where you can use those assets is subject to the license agreement. I suggest you read it carefully.