They don't provide it for download in a usable form - their download is in a .dmg Mac image, which inside is packaged as a single file .pkg.
You can't easily use it or even extract it on other platforms. Also:
> Subject to the terms of this License, you may use the Apple Font solely for creating mock-ups of user interfaces to be used in software products running on Apple’s iOS, iPadOS, macOS or tvOS operating systems, as applicable. The foregoing right includes the right to show the Apple Font in screen shots, images, mock-ups or other depictions, digital and/or print, of such software products running solely on iOS, iPadOS, macOS or tvOS. Your use of the Apple Font shall also be subject to any specific use restrictions with respect thereto as set forth in the Apple Font or Apple’s Human Interface Guidelines. You may use this Apple Font only for the purposes described in this License and only if you are a registered Apple Developer, or as otherwise expressly permitted by Apple in writing.
You can't easily use it or even extract it on other platforms. Also:
> Subject to the terms of this License, you may use the Apple Font solely for creating mock-ups of user interfaces to be used in software products running on Apple’s iOS, iPadOS, macOS or tvOS operating systems, as applicable. The foregoing right includes the right to show the Apple Font in screen shots, images, mock-ups or other depictions, digital and/or print, of such software products running solely on iOS, iPadOS, macOS or tvOS. Your use of the Apple Font shall also be subject to any specific use restrictions with respect thereto as set forth in the Apple Font or Apple’s Human Interface Guidelines. You may use this Apple Font only for the purposes described in this License and only if you are a registered Apple Developer, or as otherwise expressly permitted by Apple in writing.