In past years Messages app has featured stickers on the iPhone and iPad. Now, recently iOS 17 was announced at Apple WWDC 2023, and it comes with the enhanced feature of the sticker.
On the Message app, all the emoji are considered stickers, and all stickers are placed along with the Memoji stickers. There you can also see the downloaded stickers from the App Store. Now, with the iOS 17 feature, users can make their stickers when removing the subject from the background tool.
To make photos into stickers with iOS 17 feature you need to take a photo and then long press on the subject of the photo for highlighting it and there you see options, to make it a sticker. Select Add Sticker option and then this option transfers the subject into the sticker interface. After then you add some effects to it like a sticker outline, a puffy sticker, a glittery effect, and more.
In the above image you can see, long press on the subject or cat here then add it on the sticker interface. After adding effect if you want then you can use it sticker in your message app.