If you want the San Francisco font legally:

By following these steps, you can easily download and use the iOS-v14.ttf font in your digital projects. Enjoy the sleek and modern look of iOS 14 on your Windows, macOS, or Android device!

Standard iOS does not allow you to change the system-wide UI font without a jailbreak, but you can install fonts to use within specific apps like Pages, Keynote, or Mail 📲 How to Install Fonts on iPhone & iPad | free!