On the iPhone you can only name faces, if the Photos.app could detect the face in the photo. If you have a Mac as well and are using iCloud Photos to sync the photos between the Mac and the iPhone, the faces you added manually on your Mac will sync to the iPhone.
On the iPhone you cannot add photos to the People album, but you can add the name of the person to the caption of a photo or create a standard album named like the person. When you search for a name, the photos in the album named like the person or with the name in caption will be found as well.
On iOS 15 you can now add locations to the photos, but still not mark the faces.