Apple phone

You can pair an Apple Watch to an iPhone. If this is what you are asking, then please see the link below. If not, let us know which snartwatch you have. Also, which iPhone you have and its iOS.

Set up and pair your Apple Watch with iPhone - Apple Support

If you are trying to sync a non-Apple watch to your iPhone, contact the developer of that smartwatch and ask them

What smartwatch are you referring to that you would like to sync with your iPhone? If it is not an Apple Watch, then you will need to check with the watch's manufacturer to see if it can even sync ... and, if it does, what can it sync with an iPhone.