Look at the top-right corner of the screen -> Tap the Save button -> choose the format (photo, video, or live photo - iOS only) -> and after that:
Tap the three dots at the bottom of the page if you’d like to share or save the project in your gallery.
Android Only: Tap the arrow icon at the bottom of the page if you’d like to send the finished photo or video to your gallery.
You can also read more about how to share your finished project on social media here.