There are multiple methods for sharing photos from an iPhone to an iPad. Probably the simplest is to use iCloud service. If you own both the iPhone and the iPad, your iCloud account provides access to all your photos. So all you need to do is sync your iPhone with your iCloud account. If someone else owns the iPad, you need to grant them access via an iCloud Link. Alternatively, you can use AirDrop when the iPhone and the iPad are physically close, a USB-C cable for fast, hardware transfer, or a photo storing app, such as Mylio, that keeps all your photos in one place and allows you to share them via multiple channels (e.g., you can connect the iPhone and the iPad to the same Mylio account).