They are found when long pressing the home screen, selecting Edit and then Customize. They are called Default, Dark, Clear and Tinted. Clear and Tinted have light and dark styles.
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.
Most likely yes they would be as they would be owned by the person/org making the app, unless it is mentioned somewhere.