You can use San Francisco font in both your Apple ecosystem devices (iphone, ipad, apple watch, etc.), and your websites.
For your app, use the UIFont.systemFont APIs. You will get San Francisco font on phones for which San Francisco is the system font, which I think is iOS 9 and later.
For your website, there are tags for Safari that allow you to specify system font.
"You can access system fonts in a standards-compliant way by utilizing the system-ui family. And in Safari 13.1 we introduced new standards-based keywords to access serif, monospace, and rounded forms."
https://developer.apple.com/forums/thread/127350?answerId=614912022#614912022The restriction on San Francisco font is that if you download the font, any are for UI mockups only.
Hope this helps.
Doug Hill
https://github.com/djfitz/SFFontFeatures