Apple offers 2,400 icons for free

Can we use them in FOSS projects that don't necessarily run on Apple hardware?

No. From the license:

> Your use of Symbols obtained from Apple’s SF Font is limited to creating mock-ups of user interfaces for software products running on Apple’s iOS, iPadOS, macOS and tvOS operating systems and your use of Symbols obtained from Apple’s SF Compact Font is limited to creating mock-ups of user interfaces for software products running on Apple’s iOS, iPadOS, macOS, tvOS and watchOS operating systems. Such Apple operating systems are referred to as the “Apple Platforms”. Your use of individual Symbols shall also be subject to any specific use restrictions with respect thereto as set forth in the Apple Software or Apple’s Human Interface Guidelines.

(SF Symbol is embedded in current versions of macOS/iOS, so you don't need a special license to use it in applications which run on those platforms.)

What rights do developers have to use these on anything other than Apple products?

https://developer.apple.com/design/human-interface-guideline...

> All SF Symbols shall be considered to be system-provided images as defined in the Xcode and Apple SDKs license agreements and are subject to the terms and conditions set forth therein. You may not use SF Symbols — or glyphs that are substantially or confusingly similar — in your app icons, logos, or any other trademark-related use. Apple reserves the right to review and, in its sole discretion, require modification or discontinuance of use of any Symbol used in violation of the foregoing restrictions, and you agree to promptly comply with any such request.

Not exactly free.

IMPORTANT

All SF Symbols shall be considered to be system-provided images as defined in the Xcode and Apple SDKs license agreements and are subject to the terms and conditions set forth therein. You may not use SF Symbols — or glyphs that are substantially or confusingly similar — in your app icons, logos, or any other trademark-related use. Apple reserves the right to review and, in its sole discretion, require modification or discontinuance of use of any Symbol used in violation of the foregoing restrictions, and you agree to promptly comply with any such request.

Some symbols can’t be exported as templates for customization and can be used only to reference Apple technologies as documented below.

https://developer.apple.com/design/human-interface-guideline...

https://developer.apple.com/design/human-interface-guideline...

The installer's license agreement (yes the DMG contains an actual installer instead of just an "app") says the license for the icons is granted for use on Apple platforms. It's quite long and I haven't read it too much so there might be exceptions down the line but it's unlikely.